Shattered Ukraine town in rebel hands.
Barely a house is 
unscathed by shelling. Elderly civilians trapped by the fighting still 
cower in basements. After most Ukrainian forces pulled out Wednesday, 
the separatists are now in control.
It's
 obvious there's been a seismic shift on the battlefield. But it's not 
yet clear what the Ukrainian withdrawal means for the shaky peace deal 
that came into force Sunday. 
Will the separatists halt their offensive, satisfied with the territory they hold? Or will they be emboldened to push for more?
The
 leaders of Ukraine, Russia, France and Germany spoke Thursday by phone 
in an attempt to patch up the deal, rocked by the fierce fighting for 
control of Debaltseve, a strategic railroad hub.
They
 called for the ceasefire to be respected along the entire front line 
and for the withdrawal of heavy weaponry by both sides to create a 
buffer zone, as agreed to in Minsk, Belarus.
But skeptics may wonder if it's too late to salvage the deal, hammered out a week ago by the four leaders -- Germany's Angela Merkel, France's François Hollande, Ukraine's Petro Poroshenko and Russia's Vladimir Putin.
The ramifications for the West are huge because the 10-month-long conflict in eastern Ukraine has hiked tensions with Russia
 to a level not seen since the end of the Cold War, affecting trade and 
raising the specter of a Russian threat to Eastern Europe.
Devastation in Debaltseve
CNN's
 Nick Paton Walsh, who was able to return to Debaltseve on Thursday 
after a two-week absence, said the physical change in the town was 
astounding, with destruction on a wide scale.
Terrified
 elderly women are still taking shelter in the basement of a building 
formerly held by Ukrainian forces but now in rebel hands, he said. 
Humanitarian aid deliveries 
have started quickly, with food parcels brought in on trucks run by the 
separatists. Those still in Debaltseve -- a fraction of its former 
population of 25,000 -- are angered by what has happened to their 
hometown.
A Ukrainian tank and other 
destroyed military armor could be seen across the town, and the noise of
 machine gun fire could be heard, suggesting some Ukrainian soldiers are
 still holed up on the edges of Debaltseve, Paton Walsh said. The rumble
 of shelling also continued, mostly in the distance.
It's
 not clear yet how many civilians, separatist militants or Ukrainian 
soldiers died during the days of fighting for Debaltseve or in attempts 
to leave the town.
Ukrainian leaders 
claim their troops' withdrawal was strategic and organized, but the 
signs indicate that a vastly better equipped separatist force outgunned 
them and that they came under fire while retreating. Separatist fighters who spoke to CNN said the road out of town was still a contested area.
Kiev
 and the West accuse Russia of arming and training the separatists and 
sending Russian troops over the border to fight with them. Russia has 
steadfastly denied the claim.
Leaders: Monitors should get full access
Ukraine's Poroshenko has not hesitated to point the finger at Russia over the fighting this week in Debaltseve.
"The whole world saw that Russia doesn't adhere to its commitments," he said Wednesday after his troops' hasty retreat.
"In
 my telephone conversation with Merkel, Hollande and Putin I stressed 
that the situation around Debaltseve was in breach of Minsk accords," he
 tweeted a day later.
According to his office, he also asked for "definite guarantees in case there are further attempts to violate the agreements."
The
 Organization for Security and Cooperation in Europe, which is tasked 
with monitoring the ceasefire, has criticized the separatists for 
refusing to allow its monitors into Debaltseve.
Michael
 Bociurkiw, the OSCE spokesman in Kiev, told CNN on Thursday that the 
organization had made contact with both the Ukrainians and the 
separatists regarding access to Debaltseve. 
Separatist
 leaders in Luhansk and Donetsk said they were unable to provide safe 
passage for the OSCE monitors, he said, so "at the moment it is not 
possible for us to visit the area."
Poroshenko
 said all four leaders had agreed in their phone call that the OSCE 
should be supported in its mission and that full access should be 
ensured starting Thursday, particularly around Donetsk's airport and in 
other towns where fighting has occurred in the past hours.
Only after a full ceasefire is achieved will the drawback of heavy weapons start under direct OSCE observation, he said.
A
 statement from Hollande's office said the four leaders had agreed the 
ceasefire should be in effect on the entire front line without 
exception.
The leaders also called for 
an acceleration in the release of prisoners held by both sides, as 
agreed to under the Minsk deal. Poroshenko said this release should 
include those captured in Debaltseve.
A statement from the Kremlin said
 the leaders had stressed that the start of the implementation of the 
Minsk agreement had led to a decrease in military activity in eastern 
Ukraine and fewer civilian casualties.
"The
 leaders emphasized the need of holding a ceasefire, taking real steps 
to withdraw the heavy weapons and release of prisoners," it said.
Heightened tensions in Europe
Poroshenko
 sought Wednesday to portray his forces' retreat from Debaltseve as 
organized and orderly, disputing separatists' claims that the Ukrainian 
troops were surrounded and surrendered their arms.

U.S. ambassador to Ukraine: Putin can stop this violence.
The separatists regarded Debaltseve as already being their territory when the front lines for the ceasefire were drawn.
Hours
 after Kiev pulled its troops out of Debaltseve, Ukraine's National 
Security and Defense Council said it would ask for U.N. peacekeepers to 
be sent to eastern Ukraine.
But a spokesman for Russia's Foreign Ministry, Alexander Lukashevich, said Moscow rejects any such call.
The
 Minsk agreement is the basis for the ceasefire and resolution of the 
conflict, so there's no need for international monitoring, Lukashevich 
said at a televised briefing. He added that the government in Kiev was 
responsible for its implementation.
"We're
 convinced the full responsibility of fulfillment of the agreement lies 
only on the parties of the conflict," Lukashevich said.
Poroshenko
 said the question of deploying peacekeepers along the front line and 
the Ukraine-Russia border had been included in the Minsk agenda.
Speaking
 later Thursday in Kiev, he insisted that Russia, given its alleged 
involvement in the conflict, could not play a role in peacekeeping 
operations in Ukraine.
"Ukraine will 
not agree to a peacekeeping format, which threatens to legalize 
thousands of Russian militaries -- we already have enough such 
'peacekeepers,' " he said.
In a sign of
 the heightened tensions between Russia and other European nations, 
Britain's Royal Air Force scrambled fighter jets Wednesday after Russian
 aircraft were identified flying close to UK airspace, the Ministry of 
Defence said Thursday.
"The Russian 
planes were escorted by the RAF until they were out of the UK area of 
interest. At no time did the Russian military aircraft cross into UK 
sovereign airspace," a spokeswoman said.
UK Defence Secretary Michael Fallon told Britain's Telegraph newspaper
 Wednesday that there's a "real and present danger" that Putin would use
 murky tactics to destabilize the Baltic states of Lithuania, Latvia and
 Estonia, which border Russia on NATO's eastern flank.