MOSCOW (Reuters) - U.S. National Security Advisor Robert O'Brien will travel to Moscow to attend Russia's World War Two Victory Day celebrations in May, U.S. ambassador John Sullivan said on Thursday.

President Vladimir Putin presides over an annual parade on May 9 to commemorate the Soviet Union's World War Two victory over Nazi Germany and uses the occasion to show off the country's military hardware.

Relations between Russia and the United States are at post-Cold War lows, strained by everything from Russia's detention on spying charges of a former U.S. Marine to U.S. allegations that Moscow meddled in its 2016 election.

Russia invited U.S. President Donald Trump to attend the May 9 event, but he declined. U.S. officials said he had wanted to go, but faced pressure from his advisers not to.

National Security Advisor Robert O'Brien will lead the U.S. delegation at the event instead, Ambassador Sullivan said in a statement circulated by the embassy on Thursday.

"This high-level delegation underscores the commitment of the United States to honor the joint sacrifice of the allies, and the people of the United States and the Soviet Union who gave so much to ensure a safer world for all," it said.

The spread of the new coronavirus has raised questions over whether the May 9 parade will take place, but the Kremlin has said preparations are going ahead as normal.

(Reporting by Maria Kiselyova; writing by Tom Balmforth; editing by Andrew Osborn)