WASHINGTON (Reuters) - U.S. President Donald Trump on Tuesday blasted the investigation into alleged Russian meddling in last year's U.S. election and raised questions, without offering evidence, about Ukrainian support for his Democratic presidential rival, Hillary Clinton.

In a pair of early morning tweets, Trump also said U.S. Attorney General Jeff Sessions had "taken a very weak position" toward the Democratic presidential nominee and cited "Ukrainian efforts to sabotage Trump campaign - quietly working to boost Clinton'."

Trump did not elaborate or offer evidence about any role Ukraine may have played in the 2016 election.

The Republican president's first six months in office have been shadowed by investigations into possible Russian interference in the election and possible ties between his campaign and Moscow.

The Ukrainian president’s spokesman did not respond immediately to a request for comment. The Ukrainian Embassy in Washington denied Kiev had tried to influence the U.S. election.

"We stand by our words that the government of Ukraine didn't help any candidate in (the) election. Ukraine is proud of bipartisan support in the U.S.," the embassy posted on Twitter.

Ukraine’s permanent representative to the Council of Europe, Dmytro Kuleba, said on Twitter: "Trump writes that we interfered in the elections in the USA, while Putin says that we threaten Russia. There was a time when we were peaceful buckwheat sowers who kept themselves to themselves."

The U.S. intelligence community determined that Russia attempted to interfere in the White House race. Moscow has denied it and Trump has said his campaign did not collude with Russia.

The allegations are being investigated by U.S. congressional committees and a federal special counsel.

In the run-up to the November election, the FBI probed Clinton's handling of email as U.S. secretary of state under Democratic President Barack Obama but ultimately said no criminal charges were warranted.

(Additional reporting by Natalia Zinets in Kiev; Writing by Susan Heavey; Editing by Steve Orlofsky and James Dalgleish)