US Democratic lawmakers have demanded documents from the White House as part of their impeachment inquiry into President Donald Trump.
The documents relate to a call between Mr Trump and Ukrainian President Volodymyr Zelensky on 25 July.
In the call, Mr Trump pushed Mr Zelensky to investigate his leading Democratic political rival, Joe Biden.
The impeachment inquiry stems from the call, which was flagged up by a whistleblower in August.
The whistleblower alleged that Mr Trump used a $400m military aid package to Ukraine, which had been suspended earlier in July, as leverage to persuade Mr Zelensky. The White House released the aid in September.
Mr Trump has denied any wrongdoing, accusing his political opponents of a "witch hunt".
BBC
Russia's unmanned Luna-25 spacecraft has crashed into the Moon after spinning out of control, officials say.
Donald Trump has confirmed that he will not take part in Republican presidential debates with his rivals in the race for the White House.