MOSCOW (Reuters) - Senior Russian lawmakers said on Tuesday the resignation of U.S. President Donald Trump's national security adviser showed that efforts were being made to undermine Russian-U.S. relations, but the Kremlin declined to comment.

Michael Flynn resigned late on Monday after revelations he had discussed U.S. sanctions on Russia with the Russian ambassador to the United States before Trump took office and misled Vice President Mike Pence about the conversations.

"It's obvious that Flynn was forced to write the letter of resignation under a certain amount of pressure," Leonid Slutsky, head of the lower house of parliament's foreign affairs committee, was quoted as saying by the RIA news agency.

Flynn was a strong advocate for the need for softer foreign policy toward Russia and his departure could slow Trump's pledge to improve relations with Russian President Vladimir Putin.

"The target was Russia-U.S. relations, undermining confidence in the new U.S. administration," Slutsky said, without specifying who he thought was responsible.

Fellow lawmaker Konstantin Kosachev, who leads the upper house of parliament's international affairs committee, said the resignation could be a sign of growing anti-Russian feeling in the White House.

"Either Trump has not gained the requisite independence and he is gradually being (not unsuccessfully) backed into a corner, or Russophobia has already infected the new administration also from top to bottom," Kosachev said on social media.

The Kremlin has denied that Flynn discussed lifting sanctions with the Russian envoy and on Tuesday it declined further comment, saying the resignation was an internal matter for the United States.

"We've said everything we want to say," spokesman Dmitry Peskov told reporters on a conference call.

U.S.-Russia relations deteriorated to their worst level since the Cold War under the Obama administration, which slapped economic sanctions on Russia for its annexation of Crimea in 2014 and its involvement in Ukraine's separatist conflict.

(Reporting by Alexander Winning and Alessandra Prentice; Editing by Andrew Osborn and Louise Ireland)