Washington (AFP) - Washington vowed Monday to keep sanctions on Moscow for the annexation of Crimea as long as the peninsula remains under Russian rule, urging Russian President Vladimir Putin to end the occupation.

"On this one-year anniversary of the sham 'referendum' in Crimea, held in clear violation of Ukrainian law and the Ukrainian constitution, the United States reiterates its condemnation of a vote that was not voluntary, transparent, or democratic," State Department spokeswoman Jen Psaki said.

Russia has kicked off three days of celebrations in Crimea to mark a year since it annexed the peninsula following a referendum in a historic shift blasted by the West and Ukraine as an illegal takeover.

Pro-Russian authorities said nearly 97 percent of Crimeans voted to leave Ukraine and become part of Russia, but with no independent observers allowed the poll was widely dismissed abroad.

"Russia used its military to forcibly seize and occupy Crimea, sovereign Ukrainian territory, and then staged an illegal so-called referendum in a feeble attempt to justify its actions," Psaki told reporters.

"Over the last year, Russia has instituted repression on a mass scale in Crimea, driving out NGOs and leaving non-Russian minorities, including the Crimean tatars, to flee or go into hiding," she added.

In a separate statement, Psaki vowed that "this week, as Russia attempts to validate its cynical and calculated 'liberation' of Crimea, we reaffirm that sanctions related to Crimea will remain in place as long as the occupation continues."

She also pointed to a documentary broadcast on Sunday in which Putin explains how he gave "direct instructions" to the Russian armed forces for a swift and bloodless mission to take back the peninsula from Ukraine that had been transferred from Russian to Ukrainian jurisdiction in 1954 by Soviet leader Nikita Khrushchev.

"A year ago President Putin told the world that Russian military forces were not intervening in Crimea. He now acknowledges to the world that Russian forces did in fact intervene, and those are his own words," Psaki told reporters.

"So it certainly brings into question the credibility of claims being made today that the Russian military is not intervening in eastern Ukraine."