In a letter to the UN, the US justified the killing of Iranian General Qasem Soleimani as an act of self-defence.

Iran retaliated by firing missiles at airbases housing US forces in Iraq, causing no casualties. It also told the UN it was an act of self-defence.

Gen Soleimani was widely regarded as Iran's second most powerful man.

As head of the Revolutionary Guards' elite Quds Force, he was an architect of Iranian policy in the Middle East.

Iran's Supreme Leader Ali Khamenei described the missile attacks as a "slap in the face" for the US, and called for an end to the American presence in the region.

On Thursday the Revolutionary Guards' aerospace commander, Amir Ali Hajizadeh, said the missile strikes were the beginning of an operation that would continue.

They had not been intended kill US troops, state TV quoted him as saying, but instead were aimed at damaging US military operations.

Tehran had "used cyber-attacks to disable US plane and drone navigation systems" during the missile strikes, he said.

In a letter to the UN Security Council, US ambassador to the UN Kelly Craft said the US was ready to negotiate "with the goal of preventing further endangerment of international peace and security or escalation by the Iranian regime".

The killing of Soleimani was justified, the letter argued, under Article 51 of the UN Charter, which requires states to "immediately report" to the Security Council any measures taken in exercising the right of self-defence.

The US would take additional action "as necessary" in the Middle East to protect its personnel and interests, the letter added.

But Iran's UN ambassador Majid Takht Ravanchi said the US offer of talks was "unbelievable" while the US continued to enforce harsh economic sanctions against Iran.

US President Donald Trump has previously offered to hold talks with Iran without preconditions - and to meet President Hassan Rouhani. In September, Ayatollah Khamenei said Iran would never engage in bilateral talks, saying it was part of America's policy "to put pressure on Iran".