Ukraine says it has launched a criminal investigation into alleged illegal US surveillance of former American ambassador Marie Yovanovitch.
It follows the release of text messages involving Lev Parnas, an aide to President Donald Trump’s personal lawyer, that suggested Ms Yovanovitch was being spied on.
Mr Trump is accused of pressing Ukraine to investigate a political rival.
The claims, which he denies, are the subject of an impeachment trial.
Ms Yovanovitch, who was fired abruptly last May, has been a key witness in the proceedings against Mr Trump.
What led to Ukraine’s investigation?
Earlier this week, letters, phone records, notes and flash drives were obtained from Mr Parnas in a bid to bolster the case by US Democrats against Mr Trump at the forthcoming Senate trial.
Mr Parnas, a Ukrainian-American businessman, is an aide to Mr Trump’s personal lawyer Rudy Giuliani.
Some of the materials show Mr Parnas and Mr Giuliani discussing the removal of Ms Yovanovitch.
Several text messages, between Mr Parnas and the Republican congressional candidate Robert Hyde, appear to suggest she was being tracked in the capital Kyiv.
Mr Parnas was apparently given updates on the ambassador’s location and mobile phone use by Mr Hyde – a Republican congressional candidate in Connecticut and a Trump campaign donor.
Mr Parnas told NBC News that the only motivation to have Ms Yovanovitch removed was that she was in the way of the effort to get Ukraine to announce an investigation into former Vice-President Joe Biden.
Mr Biden is a potential Democratic rival to Mr Trump in this year’s US election.
Ms Yovanovitch has called for an investigation into the messages.
“The notion that American citizens and others were monitoring [her] movements… is disturbing,” her lawyer said.
What has Ukraine said?
The interior ministry said Ukraine’s position was not to interfere in the internal political affairs of the US but that the texts “contain information about possible violation of Ukrainian law and the Vienna Convention on Diplomatic Relations”.
It said criminal cases had been opened and US authorities had been invited to take part in the investigation.
What else did Lev Parnas say?
Mr Parnas told NBC that he went to Ukraine to put pressure on officials to investigate Mr Biden and his son, Hunter, on behalf of Mr Trump and Mr Giuliani.
He said that the president “knew exactly what was going on”.
“I wouldn’t do anything without the consent of Rudy Giuliani or the president. Why would [Ukrainian] President Zelensky’s inner circle or [Interior] Minister [Arsen] Avakov or all these people or [former] President [Petro] Poroshenko meet with me?
“Who am I? They were told to meet with me. And that’s the secret that they’re trying to keep. I was on the ground doing their work,” he added.
Documents show that Mr Parnas was in regular contact with Mr Giuliani as well as Ukrainian officials, and suggest that Mr Parnas was directly involved in trying to have President Zelensky look into Mr Biden.
One handwritten note from Mr Parnas states: “Get Zalensky [sic] to Annouce [sic] that the Biden case will be investigated.”
There is also a screenshot of a previously undisclosed letter from Mr Giuliani to Mr Zelensky, in which he asks to arrange a meeting.
Mr Parnas said President Trump “decided to” withhold military aid to increase pressure on Ukraine to investigate the Bidens’ activities.
Mr Parnas also said he told a Ukrainian official that US Vice-President Mike Pence would not attend President Zelensky’s inauguration unless there was an investigation into the Bidens.
Mr Trump has said he does not know Mr Parnas. Referring to photos of himself with Mr Parnas and another Giuliani associate, he said: “It’s possible I have a picture with them because I have a picture with everybody.”
The extent of Mr Trump’s involvement in the alleged plan to undermine Mr Biden – whose son was on the board of Ukrainian gas firm Burisma – will be examined in his impeachment trial, which is due to begin next week.
He is accused of abuse of power and obstruction of Congress but denies any wrongdoing.