(Adds no immediate reaction from Kiev)

WASHINGTON, July 25 (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.

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

Trump gave no other details or offered any evidence about any role Ukraine may have played in the 2016 U.S. presidential election, which is already under scrutiny for alleged Russian meddling and any possible ties to the Trump campaign or his associates. The Ukrainian president's spokesman did not respond immediately to a request for comment. The foreign ministry declined comment.

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."

Russia has denied any interference, and Trump has said his campaign did not collude with Moscow.

The issue has cast a cloud over Trump's first sixth months in office as Congress and the U.S. Department of Justice conduct multiple investigations.

U.S. lawmakers are also pushing new sanctions on Russia over its 2014 annexation of Crimea from Ukraine and for its alleged U.S. election interference, potentially complicating Trump's hopes of pursuing improved relations with Moscow.

In the run-up to November's election, the Federal Bureau of Investigation 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 Alison Williams and Steve Orlofsky) ((202-898-8300; Reuters Messaging: susan.heavey.thomsonreuters.com@reuters.net))