Iran's foreign ministry spokesman said Monday that the first round of talks with the U.S. in Switzerland lasted 18 hours.

Speaking to Iran's state broadcaster IRNA, Esmail Baqaei said the talks began at about 9:00 a.m. Sunday and went on until around 3:00 a.m. Monday. 

After the meeting, Iran's delegation left Switzerland and returned to Tehran.

Further discussions are expected to take place this week, but the format has not been confirmed.

Pakistani and Qatari mediators said Monday morning that Iran and the U.S. had agreed to establish a line of communication and a "de-confliction cell," following the first round of talks in Switzerland.

In a joint statement released early Monday, mediators said "encouraging progress" had been made during the "positive" Lake Lucerne Summit on Sunday.

According to the mediators, the U.S. and Iran agreed to establish a line of communication between the countries "to avoid incidents and miscommunication with the aim of safe passage for commercial vessels through the Strait of Hormuz."

Additionally, a "de-confliction cell" was also established in conjunction with Lebanon's government "to ensure the adherence of the termination of military operations in Lebanon" under the terms of the memorandum of understanding signed last week by President Trump and his Iranian counterpart, the mediators said. 

Continued fighting between Israeli forces and Iran-backed Hezbollah militants in Lebanon has threatened to derail the U.S.-Iran deal, which calls for an end to all military operations on all fronts, including Lebanon.

Iran's Foreign Minister Abbas Araghchi hailed the "progress to end Lebanon War," but said the de-confliction cell would offer the first real test of the agreement between Tehran and Washington.

Oil prices fell Monday on optimism over U.S.-Iran talks, with mediators flagging a "roadmap" to a final agreement.

Traders remained in a buoyant mood after news that the two sides had paused their conflict, which had sent energy costs soaring and stoked inflation.

There were initial jitters following reports that Iran had called off the talks over President Trump's threat to carry out more strikes if Hezbollah kept attacking Israel, but mediators Pakistan and Qatar said the talks took place in "a positive and constructive atmosphere."

Both main oil contracts fell in afternoon Asian trade, with international benchmark Brent crude down more than 1% to just under $80 a barrel.

Stock markets were mixed after a broadly positive start.

A top member of Iran's negotiating team warned American officials to "be careful with their statements" and cautioned that Iran's armed forces "are ready to respond," after President Trump threatened to strike Iran if it doesn't immediately stop Hezbollah from "causing trouble."

Iranian Parliament Speaker Mohammad Bagher Ghalibaf wrote on X: "Don't they think to themselves that if their threats had any effect, they wouldn't have reached the point of despair today? We don't count on the threats of the Americans."

The fresh saber rattling could inject new uncertainty into already delicate talks between the U.S. and Iran in Switzerland.