Russia has rejected US calls to “return” Crimea to Ukraine, saying that the strategic peninsula in the Baltic Sea is part of Russian territory.

“We don't give back our own territory. Crimea is territory belonging to the Russian Federation,” Russian Foreign Ministry spokeswoman Maria Zakharova told a news briefing on Wednesday.

Crimea declared independence from Ukraine on March 17, 2014 and formally applied to become part of Russia following a referendum, in which 96.8 percent of participants voted in favor of the move.

Following the region’s unification, the US and its allies in the European Union imposed sanctions against Russia, plunging relations with the Kremlin to a historic low.

On Tuesday, White House Press Secretary Sean Spicer said US President Donald Trump “expects the Russian government to de-escalate violence in the Ukraine and return Crimea."

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Trump repeated the call in a tweet on Wednesday morning, as he tried to contain the damage caused by the resignation of his national security adviser, Michael Flynn, over ties to Russia.

“Crimea was TAKEN by Russia during the Obama Administration. Was Obama too soft on Russia?” he said, as he blamed Flynn’s resignation on spy agencies.

When asked about Spicer’s remarks, Kremlin spokesman Dmitry Peskov said the matter was not negotiable and Russian President Vladimir Putin had already explained why Crimeans turned to Russia.

“The theme of returning Crimea will not be discussed... Russia does not discuss its territorial integrity with foreign partners,” Peskov told reporters.

Vyacheslav Volodin, speaker of the State Duma, the lower house of Russia’s parliament, echoed the same stance, telling lawmakers that any talk of Crimea's status challenges Russia's territorial integrity.

A file photo of Russian President Vladimir Putin (L) and US President Donald Trump

Russia’s UN ambassador, Vitaly Churkin, had said earlier this month that “the people of Crimea quite clearly expressed their will in a referendum.”

Trump’s remarks on Crimea came days after Putin called for the restoration of relations between Moscow and Washington, following a five-year period of "degradation."

In their phone conversation in late January, Trump and Putin both said they would like to try to mend battered US-Russia relations.