KIEV (Reuters) - Ukraine on Tuesday accused pro-Russian separatists of taking advantage of a ceasefire deal signed last September to seize more territory in the east and said Kiev would demand its return in any future peace talks.

Foreign Minister Pavlo Klimkin's charge that the rebels had seized more than 500 square km (194 square miles) beyond agreed separation lines opened up a potential new area of discord with Russia in diplomacy aimed at ending the nine-month-old conflict.

At talks under the auspices of the OSCE security watchdog in the Belarussian capital Minsk last September, Ukraine, Russia and separatist leaders agreed a plan which included a ceasefire and a withdrawal of foreign fighters and military equipment.

Subsequent meetings established the separation lines between government forces and the separatists, whom the West and Kiev say are armed by the Russian military, a charge Moscow denies.

Despite the ceasefire call, the Kiev military says more than 200 Ukrainian soldiers and hundreds of civilians have been killed since September.

The separatists have sought to dislodge government forces from the international airport in the rebel-controlled city of Donetsk, an important symbol for both sides in a conflict which has killed more than 4,800 people.

Other areas where Ukrainians say they have lost territory to the rebels are in the southeast towards Mariupol, by the Sea of Azov, and around Debaltseve, a rail hub northeast of Donetsk. INFORMATION ANALYSIS CENTER NSDC OF UKRAINE

Klimkin, speaking on the eve of a meeting in Berlin with Russia's Sergei Lavrov and the foreign ministers of Germany and France, reaffirmed that Ukraine wants to press ahead with talks only on the basis of the Minsk agreements.

"Taking advantage of the fact that our forces complied with the ceasefire, the terrorists seized very substantial territory - more than 500 square kilometers," he told a news conference.

He said Ukraine in future meetings of the so-called 'contact group' involving separatist leaders would seek a "detailed plan to return to the separation line.

"But this line is the line fixed ... in Minsk," he said.

Ukraine has used the same argument to justify its forces launching a counter-offensive at Donetsk airport, saying they were pushing back separatists to the agreed separation lines.

Russia has criticized the Kiev military's move as a "strategic mistake".

The Kremlin says President Vladimir Putin, in a letter to Ukrainian President Petro Poroshenko, has put the emphasis on the need to pull large-calibre weapons out of the conflict zone.

Ukrainian officials imply Moscow is seeking to avoid other parts of the Minsk agreements - specifically the need to withdraw foreign fighters and military equipment from Ukraine. (Additional reporting by Pavel Polityuk, Editing by Timothy Heritage)