bne IntelliNews -

Renewed four-country talks in Berlin did not yield a breakthrough in efforts to bring a full cessation of hostilities in eastern Ukraine, and fighting reignited between Ukrainian government and pro-Russian separatist forces in several locations.

The foreign ministers of Russia, Ukraine, Germany and France spent five hours trying to secure and expand agreements for the withdrawal of heavy weapons from frontline areas and to ensure effective monitoring by international observers of the ceasefire.

But the situation “remains tense because of numerous ceasefire violations”, the ministers said in a statement released after their late night consultations.

This was the fifth such meeting in the German capital since last summer to try to end the conflict that has killed more than 6,000 people in the past year. A shaky truce brokered in Minsk in February has repeatedly threated to collapse amid observed violations by both sides in recent weeks.

The talks had been "very long, very intensive and at times very contentious" given the tense situation on the ground, German Foreign Minister Frank-Walter Steinmeier told reporters. "We must ensure that the ceasefire is respected a lot more comprehensively than it was in recent days," Steinmeier said after the session with France's Laurent Fabius, Ukraine's Pavlo Klimkin and Russian Foreign Minister Sergei Lavrov. 

The talks covered the exclusion of heavy weapons, which were reportedly used in several places, including on the outskirts of the strategic port city of Mariupol, considered to be the most likely target of any renewed advance by Russian-backed rebels.

In accordance with the Minsk agreement, the sides claim to have withdrawn their heavy weaponry from frontline areas. But the Organisation for Security and Cooperation in Europe (OSCE) said "weapons with a calibre larger than 100mm were used by both sides during the fighting".

The four ministers renewed their call for the removal of such weapons and extended the appeal to include "heavy weapons below 100mm as well as all types of tanks".

OSCE monitors reported "renewed intensive fighting" on April 12 around the rebel stronghold of Donetsk, counting more than 1,100 explosions in under six hours, mainly from artillery and mortar fire.

Russia is claimed to have sent several thousand troops across the border to fight in eastern Ukraine but denies any involvement of its forces, apart from cases of “volunteers” going there while on leave.

US Ambassador to Ukraine Geoffrey Pyatt made a number of fresh claims of direct Russian involvement in comments posted on Twitter on April 14.

"Russia has command and control centers in eastern Ukraine to coordinate military operations there," Pyatt wrote, adding that Russia was conducting training for separatist forces in gunnery and artillery firing.

“Russia continues to station advanced surface-to-air missile systems near the frontline in violation of the Minsk agreements,” the ambassador added.