US Vice President Joe Biden says Russia should stick to a peace agreement on Ukraine and return the Black Sea Crimea peninsula to its western neighbor.

Biden made the comment during his latest trip to Ukraine where he told Ukrainian President Petro Poroshenko that “the US stands firmly with the people of Ukraine in the face of continued, I emphasize, continued aggression from Russia and Russian-backed separatists.”

Crimea declared independence from Ukraine on March 17, 2014 and formally applied to become part of Russia following a referendum a day earlier, in which 96.8 percent of participants voted in favor of the secession. The voter turnout in the referendum stood at 83.1 percent.

Since then, the US and some other Western countries have imposed several rounds of sanctions against Russia over accusations that Moscow has been involved in a deadly crisis in Ukraine, which broke out when Kiev launched military operations to crack down on pro-Russia forces in eastern Ukraine last year. Russia has denied the allegation.

The violence intensified in April 2014, after Kiev deployed troops to the eastern Ukrainian regions of Luhansk and Donetsk to suppress pro-Russians.

“The invasion by Russia of Crimea will not be accepted by us or by the international community. This attempted annexation is contrary to international law, is wrong, and will not be accepted in any circumstance,” Biden said.

The US VP will also meet Monday with Ukraine’s Premier Arseniy Yatsenyuk before delivering a highly anticipated address to parliament on Tuesday.

The Ukrainian crisis and Crimea’s reunion with Russia have plunged Kremlin’s relations with the West to their lowest point since the Cold War.

Thousands of people have so far lost their lives in the conflict since April 2014, according to UN figures.

Ukraine’s warring sides reached a truce deal at a summit attended by the leaders of Russia, Ukraine, France, and Germany in the Belarusian capital city of Minsk on February 11 and 12. The agreement introduced measures such as a ceasefire, which officially went into effect on February 15, the pullout of heavy weapons from the conflict zones, and constitutional reforms in Ukraine by the end of the year.

The shaky deal has, however, failed to eliminate the violence in the mainly Russian-speaking regions of eastern Ukraine, with both sides trading accusations of breaching the ceasefire agreement.