Top congressional Democrats left the door open on Sunday to pursue the impeachment of U.S. President Donald Trump, but said they would first need to complete their own investigations into whether he obstructed justice in special counsel Robert Mueller's Russia probe.

Democratic Party leaders have cautioned against impeachment just 18 months before the 2020 presidential election, although prominent liberals have called for the start of proceedings to remove Trump from office since the release on Thursday of Mueller's report.

U.S. House Judiciary Committee Chairman Jerrold Nadler, whose panel would spearhead any impeachment proceedings, said Democrats would press ahead with investigations of Trump in Congress and "see where the facts lead us."