Russia has effectively "declared war" on Britain and the United States through its actions in Crimea, Ukraine's newly-freed opposition leader Yulia Tymoshenko warned on Monday.

"Vladimir Putin is fully conscious that by declaring war (on Ukraine), he is also declaring war on the guarantors of our security, the United States and Britain," the former prime minister said in a video address posted on her website.

"They have occupied our Crimea," Tymoshenko said of the pro-Russian gunmen who have taken control of the Black Sea peninsula since Putin won a green light to use force in Ukraine.

Under a 1994 Budapest memorandum on security assurances, Washington and London agreed to guarantee Ukraine's safety in exchange for its giving up nuclear weapons.

Tymoshenko's comments coincided with a Kiev visit by British Foreign Secretary William Hague, who warned of "consequences and costs" for Russia.

Ukraine accused Russia Monday of pouring extra troops and planes into Crimea, adding to the worst standoff between Moscow and the West since the Cold War.

Kiev received no warning of the troop movements, something required by international laws on the stationing of Russia's Black Sea navy in Crimea.

Tymoshenko, who was released on February 22 after the impeachment of president Viktor Yanukovych, said that "Russia's aggression" would never have been possible had Ukraine been part of NATO.

Kiev started on the path to membership of the North Atlantic Treaty Organisation in 2008 but Yanukovych later withdrew from the process.

Tymoshenko was jailed in 2011 for seven years on contested abuse-of-power charges she says were ordered as political revenge by Yanukovych.