US President Barack Obama has thanked former secretary of state Colin Powell for announcing his support of the Iran nuclear agreement.

Obama expressed his gratitude to the veteran African-American statesman in a message posted on his Twitter account on Sunday.

The US president drew attention to Powell's rationale for supporting the deal that Republicans, who control both houses of Congress, almost unanimously oppose, because they say it gives too many concessions to Iran and threatens the security of Israel.

"Thank you, Colin, for putting your experience and expertise behind this important initiative for our country," the US president tweeted.

Iran and the P5+1 group of countries – the US, Britain, France, Russia, China, and Germany – announced the conclusion of nuclear negotiations in the Austrian capital, Vienna, on July 14.

Under the agreement, Iran has been recognized by the United Nations as a nuclear power and will continue its uranium enrichment program, but some restrictions will be placed on Iran’s nuclear program in exchange for the removal of sanctions.

Powell, a retired four-star general in the US Army, told NBC News earlier on Sunday, "It's a pretty good deal.”

He dismissed concerns put forward by opponents about independent inspectors’ ability to verify that Iran is complying with the agreement.

“I think a very vigorous verification regime has been put into place," said Powell, a Republican who served under president George W. Bush from 2001 to 2005.

"I say, we have a deal, let's see how they implement the deal. If they don't implement it, bail out. None of our options are gone," he added.

Reports say Obama is all set to achieve a major diplomatic victory on the Iran nuclear agreement when lawmakers return to Capitol Hill this week.

Democrats have enough votes to sustain the veto Obama has promised and now they are on the verge of filibustering the anti-Iran bill, preventing it from reaching the president’s desk for a veto, The Hill reported on Saturday.

So far 38 senators - 36 Democrats and two liberal-leaning independents - have announced to vote in favor of the nuclear agreement in Congress, meaning Democrats will now have enough votes to protect it. They need 41 votes in the Senate to mount a filibuster.