WASHINGTON • US Secretary of State Michael Pompeo has promised to investigate whether the ousted ambassador to Ukraine was being spied on by President Donald Trump's supporters, but said that if so, he was unaware.

House Democrats, who cited Mr Trump's dismissal of ambassador Marie Yovanovitch as they impeached him, released documents last week that suggested she was under private surveillance.

After days of the State Department not commenting, Mr Pompeo said in two interviews with conservative media that he had not previously heard of any surveillance of Ms Yovanovitch.

"We do our best to make sure that no harm will come to anyone," he told radio host Tony Katz.

"We will do everything we need to do to evaluate whether there was something that took place there. I suspect that much of what's been reported will ultimately prove wrong, but our obligation - my obligation as Secretary of State - is to make sure that we evaluate, investigate," he said.

Mr Pompeo also said he has not met Mr Lev Parnas, a Ukraine-born American who was working with Mr Trump's personal lawyer, Mr Rudy Giuliani.

The papers released by House Democrats detailed exchanges in which Ms Yovanovitch's location was discussed, indicating that she may be under watch.

Ukraine had earlier announced a probe amid indications that officials joined the alleged surveillance effort.

Mr Trump last year removed Ms Yovanovitch as ambassador and later denounced her in a call with Ukrainian President Volodymyr Zelensky that triggered his impeachment.

AGENCE FRANCE-PRESSE