Washington (CNN) After President Donald Trump publicly appealed to Iran to call him amid heightened tensions with Tehran , the White House contacted the Swiss on Thursday to share a phone number the Iranians could call the President on, according to a diplomatic source familiar with the move.

"I'd like to see them call me," Trump said of the Iranians in the White House on Thursday.

The source said the Swiss likely won't hand over the number unless the Iranians specifically ask for it and it's thought they are highly unlikely to do so. White House officials say Trump's overtures are sincere.

On Friday, an Iranian Revolutionary Guard Corps commander said Iran would not engage in talks with the "Americans," IRGC Lieutenant Commander for Political Affairs Brigadier General Yadollah Javani told the semi-official Tasnim News Agency.

"Negotiations with Americans will not take place, and Americans will not dare to take military action against us," Javani added.

The White House did not reply to request for comment on the sharing of the number.

The US and Iran do not have an official diplomatic relationship and Switzerland serves as the protecting power for the US in the country. That means they represent US interests in Iran, performing services for US citizens in the country like visa processing. They also serve as a channel for diplomacy between the two nations though there are other mechanisms for the two nations to communicate.

US has increased its military presence

Trump's public overture comes as the US has increased its military presence in the region, deploying the USS Abraham Lincoln Carrier Strike Group and a bomber task force due to what administration officials say is "specific and credible" intelligence indicating Iran poses a threat to US forces in the area.

Shortly after Trump asked Iran to call him, Secretary of State Mike Pompeo issued a statement warning that the US would issue a "swift and decisive" US response to any attack.

Iran "has engaged in an escalating series of threatening actions and statements in recent weeks," Pompeo said, echoing Pentagon and unnamed US officials. He provided no specifics about the nature or scope of that threat, but other US officials have said that they've observed Iranian forces moving missiles around on boats.

US officials have said they see no sign the Iranian threat has dissipated and on Thursday CNN reported US intelligence indicates no sign of a "decrease in Iran's posture."

Iranian officials, however, including President Hassan Rouhani, have said they have no interest in a conflict. Foreign Minister Javad Zarif has said Tehran believes some US officials are trying to provoke Iran into a war, portraying it as a trap Tehran means to avoid.

Speaking in New York last month as tensions rose, Zarif said "It is not a crisis yet, but it is a dangerous situation. Accidents, plotted accidents, are possible." Referring to a group of foreign and domestic officials that includes national security adviser John Bolton, the Iranian official added that he "wouldn't discount the B team plotting an accident anywhere in the region."

This week marks the one-year anniversary of the US pulling out of the Iran nuclear agreement.

On Wednesday Iran declared it would ease its adherence to the international nuclear pact that constrains its nuclear program in exchange for sanctions relief.

"The path we have chosen today is not the path of war, it is the path of diplomacy," Rouhani said when he made the announcement in a nationally televised address. "But diplomacy with a new language and a new logic."

The US has also taken recent measures that could make it difficult for Iran to adhere to the pact, ending non-proliferation related waivers that allowed Iran to export heavy water and low-enriched uranium to stay below levels mandated in the 2015 Joint Comprehensive Plan of Action, as the nuclear deal is formally known.

The US also introduced new sanctions on Iran this week to target their metal exports.

Iran rebuffed effort to secure hostage release

The Iranians also rebuffed a recent effort by the US to secure the release of an 82-year-old American citizen, Baquer Namazi, who has been detained in Iran for more than three years. His health is deteriorating, and the US argues that he needs to leave Iran to get medical help.

US Special Envoy for Hostage Affairs Robert O'Brien sent a letter to Iran requesting the release of Namazi at the end of April, according to sources familiar with the exchange.

If Iran lets Namazi leave, O'Brien wrote that the US would enter into a dialogue on consular affairs, which includes potential prisoners releases in both directions.

The State Department did not reply to a request for comment.

That letter was penned shortly after Zarif publicly stated that he wants a prisoner swap with the US.

"I put this offer on the table publicly now -- exchange them. All these people that are in prison -- inside the United States, on extradition requests from the United States," Zarif told an audience in New York last month. "The United States believes the charges against these people in Iran are phony, fine. Let's not discuss that, let's have an exchange. 'm ready to do it and I have the authority to do it."

But the Iranians rejected the offer made by O'Brien, according to the sources familiar with the exchange.

They said that there can be no preconditions before any potential talks. This was the second letter that O'Brien has sent to the Iranians in the last few months.

Namazi, a former UNICEF official, has been detained in Iran for more than three years. He is one of at least 6 Americans that the US government claims are wrongfully detained in Iran, including his son Siamak Namazi.