The head of the international organisation monitoring the conflict in Ukraine has said pro-Moscow separatists are constantly being re-armed, but warned that for western states to supply weapons to the Ukrainian army would risk an expansion of the war.

Lamberto Zannier, secretary general of the Organisation for Security and Co-operation in Europe (OSCE), issued the warning at the Munich security conference where the debate over supplying arms to Kiev has pitted eastern European states and US members of Congress against Germany, the UK and other western European countries. The Obama administration says it has not made up its mind.

Zannier said he supported reform and non-lethal support of the Ukrainian army, but saw huge problems in supplying lethal weaponry.

“This carries a risk with it, and the risk is that this will strengthen a narrative we are seeing already appear on the side of the separatists, that they are fighting a war against Nato and against the west,” Zannier told the Guardian in an interview in Munich.

“And if armaments appear from those countries on the Ukrainian side, that will strengthen that narrative and might even push the Russians to take a more direct role in the conflict, because it might push Russia to see itself somehow threatened.”

At the same time, Zannier said there was clear evidence that the separatist arsenal was being continually replenished, including with heavy weapons. Although OSCE monitors had not directly observed military equipment crossing over the border from Russia, because they were being denied access to much of the frontier, he said he could only assume that was where new weapons were coming from.

The secretary general of the OSCE - Europe’s 57-nation security body – said his monitors in Ukraine had been told by the separatists early on in the conflict that their weapons were drawn from abandoned Ukrainian army stockpiles. But he said that did not explain the weapons being used now.

“We saw this equipment destroyed as a result of combat operations there, but it seems to be constantly replaced. And we see the separatist side doesn’t have problems in terms of stocks of ammunition, in terms of mobility of units and therefore availability of petrol. So there seems to be an activity of constant reinforcement and bringing provisions for the combatants there,” Zannier said.

He added it was hard to say definitively where the new weapons were coming from, as the OSCE had very limited access to the Ukrainian border, although that was supposed to have been agreed by the Minsk ceasefire accord signed last September. There are 250 miles of border not under control of the Ukrainian border guard, with eight crossing points. OSCE monitors are present in just two of them, and are not permitted to patrol the “green border” between those crossing points.

“We have very limited access,” he said. “But we are very present on the other side, on the Ukrainian side. We have never seen from there military equipment going into eastern Ukraine. So we assume that any equipment going into eastern Ukraine should be coming across the border from Russia.”

Russia has consistently denied involvement in Ukraine’s conflict, though its soldiers have been arrested in the country and its military hardware has been spotted crossing the border.

OSCE has two drones intended to help watch the border from the air, but one of them crashed in the past few days, for reasons are still unknown. But even before, the use of the drones was severely limited.

“We found obstacles all the time in using them,” Zannier said. “There was electronic jamming and I was told by the company that is running the operation that is was high-spec military jamming, so we had to use counter-jamming equipment which is very sophisticated and very expensive. We also faced attempts to shoot them down. In one case we were able to record the hostile action by separatist units against our UAV. So these means of aerial control are only effective in conditions where there is respect for the ceasefire.”

He acknowledged that opposing the supply of arms to the Ukrainian army, while the separatists had no shortage of weapons, would be seen as unjust, not least by Kiev.

“I fully agree. My answer to that is we need … to see that no more arms are given to separatists in Ukraine.”

He said that if weapons supplies to the rebels continued, it could provoke western arming of Kiev, which would be the responsibility of those backing the separatists, but he added: “This still doesn’t solve the problem of the risk of the expansion of the conflict.”