WASHINGTON (NYTIMES) - The special counsel in an investigation into alleged collusion between US President Donald Trump's election campaign and Russian agents is investigating a payment made to Mr Trump's foundation by a Ukrainian steel magnate for a talk during the campaign, according to three people briefed on the matter, as part of a broader examination of streams of foreign money to Mr Trump and his associates in the years leading up to the election.

Investigators subpoenaed the Trump Organisation this year for an array of records about business with foreign nationals.

In response, the company handed over documents about a US$150,000 (S$196,701) donation that the Ukrainian billionaire, Mr Victor Pinchuk, made in September 2015 to the Donald J. Trump Foundation in exchange for a 20-minute appearance by Mr Trump that month through a video link to a conference in Kiev.

Mr Michael Cohen, the President's personal lawyer whose office and hotel room were raided on Monday in an apparently unrelated case, solicited the donation.

The contribution from Mr Pinchuk, who has sought closer ties for Ukraine to the West, was the largest the foundation received in 2015 from anyone besides Mr Trump himself.

The subpoena is among signs in recent months that Mr Mueller is interested in interactions that Mr Trump or his associates had with countries beyond Russia, though it is not clear what other payments he is scrutinising.

Mr Mueller also ordered the Trump Organisation to turn over documents, e-mails and other communications about several Russians, including some whose names have not been publicly tied to Mr Trump, according to the three people, who would not be named discussing the ongoing investigation.

The identities of the Russians were unclear.

The payment from Mr Pinchuk "is curious because it comes during a campaign and is from a foreigner and looks like an effort to buy influence", said Mr Marcus Owens, a former head of the IRS division that oversees tax-exempt organisations.

He called the donation "an unusual amount of money for such a short speech".

Mr Cohen did not respond to a request for comment.

Mr Jay Sekulow, a lawyer for the President, did not return several messages seeking comment, nor did a lawyer for the Trump Organisation.