In a meeting between Iran's President Hassan Rouhani and the emir of Qatar, both sides agreed de-escalation is the "only solution" to the regional crisis.

The responsible parties would be referred to a judicial department within the military and held accountable, the army said.

"If America has committed a crime... it should know that it will receive a decisive response," Rouhani said in a televised address, hours after Iran fired missiles at Iraqi bases housing US troops.

Iranians clutching "Down with USA" signs and portraits of Qasem Soleimani massed as supreme leader Ayatollah Ali Khamenei presided over prayers for the slain head of the Revolutionary Guards' Quds Force.

The nuclear accord between Iran and the United Nations Security Council's five permanent members -- Britain, China, France, Russia and the United States -- plus Germany was agreed in 2015.

US President Donald Trump has been actively putting out tweets, issuing warnings to Tehran, two days after a drone strike killed a top Iranian general in Iraq.

"Following the martyrdom of the glorious general haj Qasem Soleimani, I name Brigadier General Esmail Qaani as the commander of the Quds Force of the Islamic Revolutionary Guard Corps," Ayatollah Ali Khamenei said in a statement posted on his official website.

"US citizens should depart via airline while possible, and failing that, to other countries via land," the embassy said in a statement.

In a speech aired on state TV, Rouhani said the country's economy had stabilised despite punishing US measures against its senior leaders, banking and finance sectors.

Prime Minister Narendra Modi met Iranian president Hassan Rouhani on the sidelines of the high-level 74th Session of the General Assembly on Thursday amidst fresh tensions between Iran and the United States.