KIEV, June 4 - Ukraine's president told his military on Thursday to prepare for a possible "full-scale invasion" by Russia all along their joint border, a day after the worst fighting with Russian-backed separatists in months, Reuters reported.

His address in parliament was one of the first times Petro Poroshenko has used the word "invasion" to refer to Russia's behavior since the start of a separatist rebellion in the east in which the United Nations says more than 6,400 people have been killed.

Referring to a 12-hour firefight involving artillery on both sides on Wednesday when Ukraine says the rebels tried to take the town of Maryinka, Poroshenko said: "There is a colossal threat of a renewal of large-scale military operations from the side of the Russian-terrorist groups."

"The military must be ready as much for a renewal of an offensive by the enemy in the Donbass as they are for a full-scale invasion along the whole length of the border with Russia. We must be truly ready for this."

In Washington, State Department spokeswoman Marie Harf would not comment on whether the United States sees signs of a pending invasion but warned Russia to refrain from further aggressive actions in the region.

The United Nations Security Council is due to meet on Ukraine on Friday, diplomats said.

NATO said on Thursday Russia was delivering sophisticated weaponry to rebels in eastern Ukraine, renewing long-standing accusations amid the worst upsurge in fighting in months between the Kiev government's forces and pro-Russian rebels.

"We have precise information that Russia is present in eastern Ukraine and that it has delivered large quantities of heavy, advanced weapons to the separatists," NATO Secretary-General Jens Stoltenberg told reporters in Oslo.

"Artillery, anti-aircraft systems, advanced weapons systems. They have supplied more than 1,000 units of this kind to the separatists."

Moscow denies sending troops into Ukraine or helping to arm the rebels. The United Nations says more than 6,400 people have been killed in the conflict, which erupted in April 2014.

Moscow takes the side of the separatists, accusing Kiev's forces of ceasefire violations.

The Kremlin on Thursday accused Kiev of provoking new fighting with the aim of putting pressure on the European Union, which is due to decide on whether to extend economic sanctions on Russia soon.

"The Ukrainian side has taken steps to aggravate tensions many times in the past in the run-up to some major international events. This used to happen and we are seriously concerned now over the most recent manifestation of such activity," Kremlin spokesman Dmitry Peskov told journalists.

Reuters journalists have reported a Russian military buildup on the frontier in recent weeks, with thousands of troops and heavy weapons.

Ukraine and its NATO allies have long accused Russia of sending weapons and troops to fight on behalf of separatists who control part of two provinces in its east. Moscow, which seized and annexed Ukraine's Crimea peninsula last year, denies its troops are participating in the fighting in the east.

A ceasefire has mostly held for the past four months, after the separatists spurned an earlier truce to launch an advance and seize more territory for their self-proclaimed state in what Russian President Vladimir Putin calls "New Russia".

The joint border stretches for more than 2,200 km, most of it far from the area where there has been fighting.

Earlier, Ukrainian military officials said that five Ukrainian servicemen had been killed and 39 wounded in the fighting around Maryinka, which lies 23 km to the west of the rebel stronghold of Donetsk.

The rebels updated their own casualty toll to 21 dead - five civilians and 16 fighters - and blamed Ukrainian forces for indiscriminate shelling of populated areas.

The fighting went far beyond regular low-level skirmishing that has occasionally flared since the shaky ceasefire was brokered in February by the leaders of Ukraine, Russia, Germany and France.

The West and Kiev say Russia has failed to fulfill the terms of the peace deal worked out in Minsk, Belarus and say it must pull out its forces and stop arms deliveries to the separatists. (rt/ez)