The US State Department has released records relating to the Trump administration’s dealings with Ukraine.
Documents were released to the ethics watchdog American Oversight after a freedom of information request.
They mention contacts between Secretary of State Mike Pompeo and Mr Trump’s personal lawyer Rudy Giuliani, American Oversight said.
US ties with Ukraine are at the centre of an impeachment investigation against Mr Trump.
Mr Giuliani has been accused of trying to discredit former Ukraine ambassador Marie Yovanovitch while running a shadow US foreign policy on Ukraine. There have been questions over what Secretary of State Mike Pompeo knew.
“We can see why Mike Pompeo has refused to release this information to Congress. It reveals a clear paper trail from Rudy Giuliani to the Oval Office to Secretary Pompeo to facilitate Giuliani’s smear campaign against a US ambassador,” said American Oversight Executive Director Austin Evers in a statement.
“This is just the first round of disclosures. The evidence is only going to get worse for the administration as its stonewall strategy collapses in the face of court orders.”
Mr Evers said the release of the documents showed that there was no legal basis for the Trump administration to withhold records from Congress.
“That conclusively shows that the administration is engaged in obstruction of justice,” he said.
“The president and his allies should ask themselves if impeachment for obstruction is worth it if the strategy isn’t even going to be effective.”
The US president is accused of withholding US military aid to pressure Ukraine into investigating a domestic political rival.
At the heart of the inquiry is a phone call on 25 July this year between Mr Trump and Ukraine’s newly elected president.
On the call, Mr Trump urged his counterpart to look into unsubstantiated corruption claims against Democratic White House contender Joe Biden.
Mr Giuliani has been central in pushing those allegations. He has been subpoenaed for documents by impeachment investigators but has previously said he won’t co-operate with Democrats.
Mr Trump’s critics say this alleged political pressure on a vulnerable US ally amounted to abuse of power. The president strongly denies any wrongdoing.