KIEV, April 2 - Ukraine, Russia and Moscow-backed separatists are discussing possibly extending a pull-back of weapons in east Ukraine to include tanks and smaller weapons systems.

So far only weapons of over 100 mm caliber - including large artillery, heavy mortar and powerful rocket systems - have been withdrawn from the line of contact between government forces and separatist rebels under an accord reached in Minsk in February.

A key part of that agreement was a ceasefire that is still tenuously holding despite almost daily casualties.

"In order to reduce tension in the region the ... mutual withdrawal of tanks, 80-millimetre mortar and also of weapons of up to 100 mm caliber is being discussed," a spokesman for former Ukrainian president Leonid Kuchma, Kiev's representative at the peace talks, told Reuters.

Kiev sees the government-held coastal town of Mariupol on the Sea of Azov, which has a population of half a million, as particularly vulnerable to rebel attack.

A Ukrainian source close to peace talks said discussions on extending the arms withdrawal was taking place in the so-called 'contact group' involving Ukraine, Russia and the separatists under the auspices of the Organization for Security and Cooperation in Europe security watchdog.

"The aim is to widen and deepen the Minsk agreement, to make the ceasefire more effective and stable," this source said.

Rebels in Donetsk confirmed an agreement was being drawn up involving tanks, which so far have not included in any deal, being withdrawn by both sides from the line of contact.

"A document for the withdrawal of tanks ... is now being prepared," the rebel news agency DAN quoted Denis Pushilin, a senior rebel official, as saying. "None of the sides has any objection to this."

A senior OSCE official expressed concern at the fragility of the ceasefire.

"As long as guns continue to be fired and as long as substantial amounts of heavy weaponry continue to be concentrated in defensive and offensive positions, achieving a comprehensive and sustainable ceasefire will be difficult," Alexander Hug, the OSCE's deputy chief monitor in Ukraine, told journalists in Donetsk. (rt/ez)