The federal indictment of two men -- one a Turkish citizen -- for acting as illegal agents of the Turkish government in the US adds another irritant to an already fractious relationship.

Bijan Rafiekian and Kamil Ekim Alptekin were accused Monday of being involved in a conspiracy "to covertly influence U.S. politicians and public opinion against a Turkish citizen living in the United States," the Justice Department said. According to the indictment, the two conspired to "delegitimize the Turkish citizen ... with the goal of obtaining his extradition."

That Turkish citizen is the 77-year-old cleric Fethullah Gulen, accused by the Turkish government of planning an abortive coup in 2016, a charge he has persistently denied. For nearly three years Turkey has demanded Gulen's extradition from the US, where he has been living for almost two decades.

Turkey's Foreign Minister Mevlut Cavusoglu said Sunday that US President Donald Trump told Turkish President Recep Tayyip Erdogan at the G20 summit in Argentina this month that the US was "working on" the extradition of Gulen.

Cavusoglu claimed Sunday during an interview in Doha that the FBI had evidence that the cleric's organization, known as FETO, "had been violating US laws, including tax fraud, visa fraud and also some other illegal activities."

The FBI declined to comment, and Cavusoglu's comments don't square with recent statements from the US State and Justice Departments. Last month, the State Department said only that the US had received multiple requests from the Turkish government and continued to evaluate materials presented.

Legal analysts say that Gulen's lawyers would have ample grounds for challenging any extradition proceedings, including a "political offense" clause in the 1979 extradition treaty between the two countries.

Those grounds may have been made stronger by Monday's statement from the Justice Department, which said that "not only did Turkish cabinet-level officials approve the budget for the project, but Alptekin provided the Turkish officials updates on the work and relayed their directions on the work to Rafiekian."

The Justice Department said Rafiekian, 66, lived in San Juan Capistrano, California, while Alptekin, 41, lived in Istanbul. If convicted, each faces a maximum penalty of five years in prison for the conspiracy charge and 10 years in prison for a charge of acting as an agent of a foreign government.

Thorny relations between US and Turkey

Despite being NATO allies, Turkey and the US have in recent years been at odds on a variety of issues besides Gulen: policy towards Syria, the arrest of an American pastor in 2016 and retaliatory US sanctions against senior Turkish ministers this year, allegations by some Turkish officials that the US was complicit in the 2016 coup attempt and the decision by Turkey (a NATO member) to buy advanced Russian radar.

Turkey has also been irritated by what it regards as a lenient approach by the Trump Administration towards Saudi Arabia in the wake of the killing of journalist Jamal Khashoggi.

After a calmer spell which saw the release of American pastor Andrew Brunson in October after two years in jail and under house arrest, US-Turkish relations look to be taking another dive.

On Sunday, Cavusoglu described Brunson as a CIA agent, but said the Gulen issue and "US support to YPG/PKK in Syria, which are posing threat to our national security," were more serious issues.

The YPG has been Washington's main ally on the ground in northeastern Syria against ISIS but is regarded as a terrorist group by Turkey, indistinguishable from Kurdish PKK separatists inside Turkey.

Last week, with support from US airstrikes, YPG fighters drove ISIS out of the town of Hajin.

The US supply of weapons to the Syrian Democratic Forces, of which the YPG is the leading element, has long infuriated Turkey. Cavusoglu asked Sunday: "Is America the shelter of PKK terrorist organization?....Why did they give that many weapons - more than 20,000 truckloads of weapons?"

Erdogan's language aimed at his political base

President Erdogan has repeatedly threatened to launch another incursion into northern Syria to attack the YPG.

On Friday he said Turkey was determined "to bring peace" to what he called "the terror swamp" controlled by the YPG. He warned the US "should eradicate these terrorist organizations, or else we will do it," language he repeated Monday.

The Pentagon responded that "unilateral military action into northeast Syria by any party, particularly as U.S. personnel may be present or in the vicinity, is of grave concern."

US forces currently patrol a deconfliction line around the town of Manbij, from which Turkey is demanding the YPG is expelled.

In his Doha comments, Cavusoglu also claimed that "President Trump is now considering to leave Syria -- again." Back in March, Trump suggested the 2,000 US forces deployed in Syria in support of the Syrian Democratic Forces might soon leave.

But they haven't, and US officials have recently suggested they will stay for as long as necessary to stamp out ISIS, which still holds on to enclaves along the Syria-Iraq border.

Much of Erdogan's aggressive language is aimed at his political base at home. He has often suggested a Turkish offensive inside Syria is imminent. But since the limited incursion into Afrin at the beginning of this year, the Turkish military has been more preoccupied with guarding the ceasefire in the Syrian province of Idlib.

The US and Turkey have agreed to make progress on a "roadmap" for contested parts of northern Syria by the end of the year, but the tone of Erdogan and Cavusoglu's remarks over the past few days suggest that on a range of issues, Turkey's patience is being sorely tried. Monday's indictment will do nothing to improve the mood music.