President Vladimir Putin on Tuesday signed a treaty claiming the Black Sea region of Crimea as Russian territory as Ukraine warned the showdown had entered a "military stage" after soldiers were killed on both sides.

The treaty signing was conducted at lightning speed in the Kremlin in a defiant expansion of Russia's post-Soviet borders that has plunged relations with the West to a new post-Cold War low.

Britain warned Tuesday that the West and Russia faced a changed relationship in the coming years, as London suspended all bilateral military cooperation and halted arms exports to Russia.

Foreign Secretary William Hague said President Vladimir Putin had chosen the "route of isolation" by signing a treaty annexing Crimea just two days after a hastily arranged referendum on the breakaway peninsula.

President Vladimir Putin on Tuesday signed a treaty making Crimea part of Russia, in a historic redrawing of Russia's borders after he declared that the Black Sea region has always been "in the hearts" of his countrymen.

In a fast-moving sequence of events following Crimea's controversial secession referendum on Sunday, the Kremlin said Crimea was now considered part of Russia and no longer Ukrainian territory, shrugging off strong international objections.

The United States on Monday imposed financial sanctions on seven top Russian government officials and lawmakers to punish Russia's incursion into Crimea.

In a new executive order, President Barack Obama also imposed sanctions against former Ukrainian President Viktor Yanukovych and one of his top advisors and two top "separatist" leaders in Crimea.

Crimea's regional assembly on Monday declared independence from Ukraine and applied to join Russia, saying all Ukrainian state property on the peninsula would be nationalized.

Later on Monday, Russian President Vladimir Putin signed a decree formally recognizing Crimea as an independent state following its vote to secede from Ukraine and join the Russian Federation.

Russian President Vladimir Putin on Sunday told U.S. President Barack Obama in telephone talks that the referendum on the Ukrainian region of Crimea joining Russia was "completely in line with the norms of international law," the Kremlin said.

Putin also told the U.S. president that any Organization for Security and Cooperation in Europe (OSCE) monitoring mission in Ukraine should cover "all Ukrainian regions" and not just Crimea.

Ukraine's defense minister said Sunday that Russia had agreed to temporarily lift its blockade of Ukraine's military bases in Crimea in order to ease tensions surrounding the peninsula's secession referendum, as Moscow said it would respect the “choice” of the flashpoint region's residents.

"Agreements have already been reached between our commanders... on there being no attempts to blockade our military installations until March 21," Interfax quoted Defense Minister Igor Tenyukh as saying.

Thousands of people rallied in central Moscow Saturday in protest at Russia's intervention in Ukraine, a day before the Crimean peninsula is expected to vote on switching to Kremlin rule.

Waving Ukrainian flags and shouting slogans heard during the Maidan protests in Kiev, the demonstrators urged Russian President Vladimir Putin to pull troops back.

Ukraine braced Saturday for a highly-charged breakaway vote in Crimea which is likely to see the peninsula annexed by Russia despite the threat of stiff sanctions, and push East-West tensions to breaking point.

An eleventh-hour diplomatic push by the United States to stop the referendum from going ahead failed on Friday, with Moscow refusing to make any decisions until after Crimea votes on a split from Ukraine on Sunday.

Since his return to the Kremlin, Vladimir Putin has grown more powerful than ever before. The strongman, who has ruled Russia for the past 14 years, has muzzled his opponents, check-mated the West on Syria and is now on the verge of annexing Crimea.

On Friday, the Kremlin upped the ante further, hinting it could move forces beyond Ukraine's peninsula to protect his compatriots.