Russia responded has angrily to news that US senators had passed a bill calling for fresh sanctions against Moscow and the supply of lethal military aid to Ukraine.
"Undoubtedly, we will not be able to leave this without a response," deputy foreign minister Sergey Ryabkov told Interfax news agency on Saturday ahead of a meeting between the Russian and US foreign ministers.
Ukraine welcomed the bill, that would allow the US to provide lethal military assistance to the country.
The bill, passed late on Thursday approved in Congress on Friday will now be sent to US President Barack Obama.
It opens the way for up to $350m of US military hardware to be sent to Ukraine.
Ukraine has been fighting an eight-month war against pro-Russian separatists in its east.
The bill also threatens fresh sanctions against Russia, whose economy is weakening under previous rounds of Western sanctions and a collapse in oil prices.
John Kerry, US secretary of state, was due to meet Sergey Lavrov, Russia's foreign minister, in Rome on Monday amid the toughening US 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," Alexander Lukashevich, a Foreign Ministry spokesman, said.
"US legislators are following in the footsteps of the Barack Obama administration by showing great zeal in destroying the framework of cooperation."
'Historic decision' welcomed
Ukrainian politicians, 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 Obama 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.
US politicians, 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.
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 continues to deny the accusations.