Kiev (AFP) - Dozens of tanks and truckloads of soldiers have crossed from Russia into Kremlin-backed rebel territory, Ukraine said, though neither NATO nor the US were able to verify the claim.
Meanwhile the Ukraine military said Saturday one Ukrainian soldier has been killed and 15 wounded in fighting with pro-Russian rebels over the past 24 hours, as clashes continued to break a nominal truce.
One paratrooper was shot dead by a sniper in the ruins of Donetsk's international airport, where Ukrainian forces are almost surrounded by rebels, the military said in a statement.
Another 15 servicemen were wounded as government positions around the conflict zone came under repeated bombardment, the statement said.
The allegations that Moscow is stepping up reinforcements for the insurgents stoked fears that both sides could slide into a return to all-out fighting.
A column of 32 tanks, 16 howitzer cannons and 30 trucks carrying troops and equipment crossed the border into the separatist-held Lugansk region Thursday, Ukrainian military spokesman Andriy Lysenko said, adding that another convoy including three mobile radar stations had also entered the same area.
US State Department spokeswoman Jen Psaki said Russian battle tanks, armoured vehicles and cargo trucks had been seen Thursday about 25 kilometres (15 miles) from the border, but added there was no "independent confirmation" of the reports.
"If confirmed, the United States condemns this most recent incursion into Ukrainian territory," she said. "It would be another blatant violation of the Minsk agreement signed by Russia and the separatists."
The Russian defence ministry said Friday that a string of Western accusations concerning troop movements around the Ukraine border were "untrue".
- Rare chance for dialogue -
In a rare chance for dialogue, Russian Foreign Minister Sergei Lavrov will meet his US counterpart John Kerry Saturday ahead of the APEC summit in Beijing next week, Russian news agencies reported.
But in a sign of how far relations have slumped, the Kremlin ruled out an official sit-down between President Vladimir Putin and US leader Barack Obama.
The Kremlin strongman has agreed to an uncomfortable meeting with Australian Prime Minister Tony Abbott, who has promised to confront Putin over the deaths of Australians onboard the Malaysia Airlines jet shot down over rebel territory in July.
Dutch investigators have recovered more human remains from the crash site, a rebel official said Friday, although it is too early to tell if they are some of the 298 victims from the downed plane or combatants killed in fighting.
Relations between Russia and the United States have become the iciest since the end of the Cold War after Moscow annexed Ukraine's Crimean peninsula in March.
Russia is blamed for stirring the war between Kiev and the pro-Moscow separatists, including by sending its own troops across the border to eastern Ukraine, where over 4,000 people have been killed by fighting since April.
Kerry on Wednesday warned Russia that sanctions pressure could be intensified if Moscow does not show commitment to the ceasefire agreement signed in September in the Belarussian capital Minsk.
The United States and European Union have already slapped tough sanctions on Moscow and Poland's foreign minister has warned of a new Iron Curtain falling across Europe.
Top EU diplomat Federica Mogherini said Thursday that the 28-nation bloc would review sanctions on Russia in 10 days, with pressure mounting to add to the punitive measures after Moscow endorsed the rebel elections.
- Crumbling peace plan -
While the September Ukraine truce agreement has seen full-scale confrontations halt along most of the front line, shelling has continued at flashpoints around the industrial east.
Ukrainian President Petro Poroshenko warned German Chancellor Angela Merkel in a phone call of an escalation in the conflict following "significant departures" from the agreed peace plan, his office said.
The rebels held leadership elections on Sunday, defying Kiev with a move that sought to formalise their control over the separatist-held territory.
In response, Ukraine's border guards announced obligatory passport controls around the rebel-held areas on Thursday, effectively setting up a de facto border despite Kiev's insistence that it has not given up on reclaiming sovereignty.
That move dovetailed with a government decision to sever state subsidies worth some $2.4 billion (1.8 billion euros) each year to the guerrilla regions.
In the wake of Sunday's rebel polls, Poroshenko also said separatists had "torpedoed" a government proposal to give them autonomy and he ordered troops to reinforce frontline cities.
Moscow's foreign ministry on Friday appeared keen to pull back slightly by specifying it "respected" -- but had not officially recognised -- the results of the vote, although it was not clear whether the clarification would be enough to ease Western criticism.
The financial isolation over the Ukraine crisis -- along with falling oil prices -- has hammered Russia's flagging economy. The ruble plunged early on Friday by over three percent to a new record low of over 60 to the euro before recovering to around 57.
As concerns mounted over the volatile currency, Russia's central bank went back on a pledge Wednesday to limit interventions by saying it was now willing to prop up the ruble.