The US is sending the aircraft carrier Theodore Roosevelt and extra marines to the Middle East. The ship left San Diego on Sunday, about seven months after war broke out between the US and Iran. The move comes as President Donald Trump threatens to resume strikes on Iran after the mid-term elections in early November.

The Theodore Roosevelt will replace the George Washington, which is based in Japan and was sent to the Central Command area last month. The George Washington had taken over from the Abraham Lincoln, which was pulled out after reports of serious mental health problems among its crew.

US ships are keeping up a naval blockade off Iran's coast. Washington is also working to get more oil flowing through the region.

On Wednesday evening, Trump repeated that Iran must not get a nuclear weapon, that the war will end soon, and that Iran is in very bad shape. "I don't know if they'll surrender right now, but they will surrender," the president told reporters.

US officials say the stalled talks have left both sides thinking a new clash is more likely, after they traded threats this week. The same officials believe Trump could order full-scale military action again once the election is over.

On Tuesday, Mohsen Rezaei, secretary of Iran's Supreme National Security Council, said the US administration, unable to act, had turned to an air blockade of Iran instead. He said that throughout history many foreigners have come and gone, while Iran and the countries of the region have stayed.

"The president of the United States is trapped. He can't negotiate, and he can't go to war," Rezaei said.

Qatar is pushing for a breakthrough in US-Iran talks to end the war and is stepping back into its role as mediator. Doha earlier helped draft the June memorandum of understanding that led to a ceasefire, but the truce fell apart soon after.

Qatar's efforts have not led to real progress so far. Officials in Washington expect Doha to keep up its diplomatic push despite widespread frustration.

Tension is also rising in the Strait of Hormuz, as well as over worsening clashes between Saudi Arabia and the Houthis in Yemen.