The Trump administration placed sanctions on Iranian Foreign Minister Javad Zarif on Wednesday.

The action blocks Zarif from accessing the U.S. financial system and is an escalation in President Donald Trump’s campaign of “maximum pressure” against the Islamic Republic.

Zarif “is the regime’s primary spokesperson around the world. The United States is sending a clear message to the Iranian regime that its recent behavior is completely unacceptable,” Treasury Secretary Steven Mnuchin said in a statement on the measure.

Zarif responded on Twitter within minutes.

“It has no effect on me or my family as I have no property or interests outside Iran,” he wrote. “Thank you for considering me such a huge threat to your agenda.”

While officials say Trump is keen to negotiate with Tehran and does not want a war, the hardening line suggests tension will continue ― particularly given that Zarif is Iran’s top diplomat and has previously been a key interlocutor with the U.S.

“If our position is really that we want to negotiate with Iran ... maybe we shouldn’t sanction their chief negotiator. Just sayin,” Sen. Chris Murphy (D-Conn.) posted on Twitter in response to the news.

Still, the development comes the day after officials told The Washington Post that the president will for now sustain one critical element of the 2015 global nuclear agreement with Iran despite his opposition to the deal, a potential signal of interest in dialogue. The same day, officials from the United Arab Emirates, one of Trump’s closest partners in the Middle East, met with counterparts from Iran.

Sanctioning Zarif will not affect the chance to negotiate because the Trump team does not see him as “a significant decision-maker,” an administration official said in a White House call with reporters ahead of the announcement. The minister’s function is covering up Iranian atrocities like crackdowns on the country’s LGBT community and the U.S. would like to deal with someone close to Iranian Supreme Leader Ali Khamenei, the official added.