US officials pressured their Ukrainian counterparts to launch investigations that could benefit President Donald Trump's personal political agenda in exchange for a meeting between the two countries' leaders, a cache of diplomatic texts released late on Thursday showed. The exchanges were released by Democrats in the House of Representatives as part of an impeachment investigation to determine whether Trump pressed for Ukraine to probe former Vice President Joe Biden and his son, Hunter Biden, in connection with Ukrainian gas company Burisma.

Biden is a leading contender for the 2020 Democratic presidential nomination. His son was on the board of Burisma for a number of years. Kurt Volker, who resigned a week ago as Trump's special representative to Ukraine, provided the messages to members of the House and staff of the House Foreign Affairs, Intelligence and Oversight committees in a closed-door meeting earlier on Thursday.

Democrats are focusing on a July 25 telephone call between Trump and Ukrainian President Volodymyr Zelenskiy in which the Republican president urges Zelenskiy to investigate Burisma and the Bidens. In the hours before that call, Volker told one adviser to the Ukrainian president that a meeting between the countries' two leaders was tied to Kiev's agreement to investigate the 2016 US election, according to the committees.

"Heard from the White House - assuming President Z convinces trump he will investigate/'get to the bottom of what happened' in 2016, we will nail down date for visit to Washington," Volker wrote.