Secretary of State Mike Pompeo said Wednesday he would be ready to testify in President Donald Trump's impeachment trial if legally obliged to, as political squabbles over the process intensified in Washington.

"If I am legally required to testify, as I've said before, I'll be happy to do it," Pompeo told reporters in Jamaica.

Pompeo said he had not been tracking the impeachment proceedings while on a Western Hemisphere tour this week.

The Republican-controlled U.S. Senate voted early on Wednesday on party lines to approve the rules for Trump's impeachment trial, rejecting Democratic efforts to obtain evidence and ensure witnesses are heard.

As the trial began in earnest, a top Democratic lawmaker said there was "overwhelming" evidence of wrongdoing and Trump's chief legal defender argued the Democratic case was a baseless effort to overturn the 2016 election.