PODGORICA, Montenegro (AP) — U.S. Secretary of State Mike Pompeo is visiting Montenegro to show support for the NATO ally as a Ukraine-focused impeachment inquiry into President Donald Trump picks up steam.

Pompeo arrived in Podgorica on Friday, a day after one of his former top aides told congressional investigators about efforts to press Ukraine’s government to open a corruption probe that could have targeted former Vice President Joe Biden’s son. Pompeo did not speak to reporters aboard his plane during the brief flight from Rome.

Pompeo this week acknowledged for the first time that he had been on a July 25 phone call between Trump and Ukraine’s president during which an investigation was raised. He said the call was part of a broader effort to get Ukraine to crack down on corruption.