Ukraine welcomed a US bill that would allow Washington to provide lethal military assistance to the embattled country, but Russia expressed outrage at the "openly confrontational" legislation.
The bill -- passed late on Thursday and due to get final approval in Congress on Friday before being sent to US President Barack Obama -- opens the way for up to $350 million (280 million euros') worth of US military hardware to be sent to Ukraine, which has been fighting an eight-month war against Kremlin-backed separatists in its east.
It also threatens fresh sanctions against Russia, whose economy is crumbling under previous rounds of Western sanctions and a collapse in oil prices.
US Secretary of State John Kerry is set to meet Russian Foreign Minister Sergei Lavrov in Rome on Monday amid the toughening American response.
Russia's foreign ministry said the new US legislation put a "powerful bomb" under US-Russia bilateral ties.
"The openly confrontational nature of the Ukraine Freedom Support Act approved by both houses of the US Congress without debate and proper voting cannot cause anything but deep regret," said ministry spokesman Alexander Lukashevich.
A woman looks at a US President Barack Obama doll at a street market in Simferopol, Crimea, on December 12, 2014.
"US legislators are following in the footsteps of the Barack Obama administration by showing great zeal in destroying the framework of cooperation," he said.
Kiev lawmakers, though, hailed the US move as a "historic decision". They have long been pressing the West to provide military support to their beleaguered army, but have so far received only non-lethal equipment.
Obama, who has resisted sending arms to Ukraine, will have to decide whether to promulgate the Ukraine Freedom Support Act, which allows for the delivery of anti-tank and anti-armour weapons, radar, surveillance drones and communications equipment to Ukraine.
It is far from certain that the US president will back the bill. There is little appetite in Western governments for a step that could see them drawn into a proxy war with Russia.
Asked whether Obama would sign it, White House spokesman Josh Earnest said: "We are looking at it right now."
US lawmakers, however, appeared determined to force Obama's hand against Russia. Senators added a clause in the bill that would grant "major non-NATO ally" status to Ukraine, along with pro-Western Georgia and Moldova.
Employees of the Russian Emergency Ministry unload electrical equipment, part of a Russian convoy carrying humanitarian aid for the eastern Ukrainian Donetsk region, in the city of Makiyivka near Donetsk, on December 12, 2014.
Russia is concerned at what it sees as NATO's creeping influence along its western borders.
Ukraine and the West accuse Russia of sending regular troops to back separatists in eastern Ukraine in a conflict that has claimed more than 4,300 lives since it broke out in April.
Russia denies the accusations.
'They were ambushed'
Ukrainian President Petro Poroshenko, meanwhile, said a ceasefire put in place on Tuesday -- the fourth since the conflict began in April -- was "real" but still remained fragile.
"Today is the first 24 hours for seven months... when we have a real ceasefire in Ukraine," Poroshenko said during a trip to Australia.
"Everything is so fragile. But I pray that we should continue this process."
Underlining his concerns, there were reports on Friday that two volunteer fighters with the pro-Kiev "Azov regiment" and two rebels were killed in Pavlopil, 70 kilometres (45 miles) south of the main rebel stronghold of Donetsk.
"They were ambushed. Their car was blown up by a landmine," Azov spokesman Oleksandr Alferov told AFP. He added that the two rebels were killed in an ensuing firefight.
Ukraine's Defence Minister Stepan Poltorak said the government plans to conscript an additional 40,000 soldiers and train a further 10,500 professional soldiers next year -- taking the total number of armed forces to 250,000.
Residents queue to receive humanitarian aid in the eastern Ukrainian city of Donetsk on December 11, 2014.
The country scrapped conscription in October 2013 only to be forced to reinstate it in May this year, in the wake of Russia's annexation of Crimea and the outbreak of the Moscow-backed insurgency in the east.
'No interest in dialogue'
Russia denies sending military aid over the border and argues that any of its troops fighting in eastern Ukraine are volunteers on vacation. But its claims have been roundly rejected by Kiev and the West.
NATO accused Russia on Friday of refusing any "genuine dialogue" about its increased military activity across eastern Europe.
The US-led alliance has reported an upsurge in Russian military flights, especially near its Baltic state members, but also further afield around Norway, Britain and far south into the Atlantic.
Poroshenko said the conflict in his homeland was not just about Ukraine's independence and territorial integrity.
"This is a war for freedom, global freedom. This is a war for democracy, global democracy and this is the war for security, global security," he said.
Poroshenko added that he believed the Ukrainian conflict was a "very emotional" matter for Russian President Vladimir Putin, but that peace was possible.
"I avoid the term 'we will win war'. But I can use the term 'we will win peace'," he said.