The Pentagon says Russia has increased its troop deployment along the border with Ukraine in the past 24 hours.
Pentagon spokesman John Kirby told reporters the number of soldiers Russian President Vladimir Putin had assembled at border numbered "well north of 100,000 and it continues to grow".
"Every day he adds to his options, every day he adds to his capabilities," Mr Kirby said.
He said Russia had also moved up to 30,000 troops, two battalions of S-400 surface-to-air missile systems and numerous fighter jets into Belarus for joint training exercises with the Belarusian army.
Images from US-based Maxar Technologies showed military units armed with missiles, multiple rocket launchers and attack aircraft had been deployed in Belarus at three locations close to the border with Ukraine.
The White House said Russia's vast military exercises in Belarus were increasing tensions with Ukraine.
"As we look at the preparation for these military exercises, again we see this as certainly more an escalatory and not a de-escalatory action," White House spokesperson Jen Psaki said.
Another group of the Russian navy's large landing ships passed through Bosphorus strait of Istanbul into the Black Sea on Wednesday following three sailing through a day earlier.
Citing Russia's Defence Ministry, Russian news agency Interfax on Tuesday said six Russian warships were heading to the Black Sea from the Mediterranean for naval drills.
It said it was a pre-planned movement of military resources.
Russia announced last month its navy would stage a sweeping set of exercises involving all its fleets in January and February, from the Pacific to the Atlantic — another show of strength during a surge of military activity during the stand-off with the West over Ukraine.
US military shipment arrives in Romania
The US army also continues to build up defences in the region, with the first convoys carrying US military equipment and vehicles to Romania arriving late on Wednesday.
Some 1,000 US troops are being deployed to the country in the coming days.
Troops from the 2nd Cavalry Regiment were deployed from Vilseck, Germany, and crossed through Romania's western border point of Nadlac in Arad County at about 10pm local time.
They will join some 900 US service members that are already stationed in Romania.
Romania, a NATO member since 2004, shares more than 600 kilometres of border with Ukraine, to the north.
Moscow has said it wants guarantees from the West that Ukraine and other former Soviet nations will not join NATO.
The US and NATO have rejected Moscow's demands.
UK readies 1,000 troops
Britain has ordered 1,000 troops to be ready to provide support in the event of a humanitarian crisis caused by any Russian aggression
It comes ahead of a visit by Prime Minister Boris Johnson to the leaders of NATO and Poland on Thursday.
Mr Johnson will go to Brussels and Warsaw to stress the need to hold firm on NATO's principles, and discuss ways that Britain can provide military support while Russia amasses its troops near Ukraine's border.
Mr Johnson's trip is part of a wave of recent international diplomacy taking place over the crisis.
French President Emmanuel Macron met Mr Putin earlier this week and US Vice-President Kamala Harris is due to have in-person meetings with US allies and partners at the Munich Security Conference next week.
Britain's Foreign Secretary and Defence Secretary are also due in Moscow this week for talks with their counterparts.
"The UK remains unwavering in our commitment to European security," Mr Johnson said in a statement.
Britain said on Monday it would send a further 350 troops to Poland, after it sent 100 troops last year to help with a migrant crisis at its border with Belarus.
Mr Johnson's office repeated on Wednesday that any further military incursion of Ukraine by Russia would likely create the mass forced displacement of people on Europe's border, affecting countries such as Poland and Lithuania.
In 2014, Russia annexed the Crimean Peninsula from Ukraine.
Mr Johnson's office said the Prime Minister would also discuss with NATO secretary-general Jens Stoltenberg the UK's offer to bolster the alliance's defences, including a doubling of troops in Estonia, more RAF jets in southern Europe, and the sailing of both the Trent patrol vessel and a Type 45 destroyer to the eastern Mediterranean.
ABC/wires
