Ukrainian Foreign Minister Pavlo Klimkin on Tuesday cast doubt on hopes that a UN-backed truce with rebels will stick, as he called for an expansion of international monitoring.

"The situation on the ground is very difficult and tense despite a declared ceasefire. We still have many shells thrown by terrorists" in eastern Ukraine, Klimkin told reporters in Tokyo.

"There was always a problem of lack of trust in relations between Ukraine and Russia ... we can't rely on any kinds of agreements between us and Russians.

"And exactly because of that we need [a] consistent position of the whole international community for defending Ukraine peace and Ukrainian territorial sovereignty and territorial integrity."

Klimkin - who is in Japan for meetings with Prime Minister Shinzo Abe and his counterpart Fumio Kishida - made the comments a day after the US and Russian foreign ministers John Kerry and Sergei Lavrov expressed cautious optimism following talks in Geneva.

Their meeting was part of efforts to end the fighting in Ukraine, where the UN says more than 6,000 people have died in less than a year.

Kerry and Lavrov both said a February 15 cease-fire was on the right track, despite repeated breaches.

Ukraine's army said Tuesday that three soldiers had been killed and nine wounded in the last 24 hours.

Both sides have begun to pull back some heavy weaponry from the frontline, but monitors from the Organization for Security and Cooperation in Europe have said it is too early to confirm a full pullback.