Paul Manafort, campaign chairman for Republican presidential nominee Donald Trump, has resigned just two months after taking the helm.
He first took control of Mr Trump’s campaign after former campaign manager Corey Lewandowski was fired in June.
We had concerns that Iran may renege on the prisoner release. We sought to retain maximum leverage until after the American citizens were releasedState Department spokesman John Kirby
The 67-year-old veteran political strategist has come under fire for his ties to Russian-backed, former Ukrainian President Viktor Yanukovych. But it is not yet clear why he left the team, which was overhauled earlier this week with the addition of a new campaign chief executive and a new campaign manager.
Mr Trump has come under pressure in recent weeks after a series of controversial remarks and falling poll numbers.
Meanwhile the Obama administration is facing a storm of Republican criticism after acknowledging that a $400 million cash payment to Iran seven months ago was contingent on the release of a group of American prisoners.
The explanation was the first time the US had so clearly linked the two events, which critics have painted as a hostage-ransom arrangement.
Repeating past administration statements, State Department spokesman John Kirby said the negotiations to return the Iranian money from a 1970s account to buy US military equipment were conducted separately from talks to free four US citizens in Iran. But unlike with past descriptions, he said the US withheld the delivery of the cash as leverage until Iran permitted the Americans to leave.
“We had concerns that Iran may renege on the prisoner release,” Kirby said, citing delays and mutual mistrust between countries that severed diplomatic relations 36 years ago. As a result, he explained, the US “of course sought to retain maximum leverage until after the American citizens were released.”
Both events occurred January 17, fuelling suspicions from Republican lawmakers and accusations from GOP presidential nominee Donald Trump of a quid pro quo that undermined America’s longstanding opposition to ransom payments.
In a speech on Thursday night in Charlotte, North Carolina, Trump accused President Barack Obama of lying.