Iranian chief of staff Maj. Gen. Mohammad Hossein Baqeri warned on Thursday that the United States could “face the most severe reactions,” a day after U.S. President Donald Trump promised that “Iran will pay a very heavy price” for any attacks on Iraq.

The two countries have squared off in Iraq several times over the past few months, coming to the brink of war in January. Tensions seem to be rising again after a series of clashes killed two American troops and left several more wounded in March. Iran has denied responsibility for the deadly attacks even as it warns the United States against retaliation.

“U.S. military activities in Iraq and the Persian Gulf region have increased,” Baqeri told the state-run Islamic Republic News Agency on Thursday. “We keep a close eye on these activities, and our nation's forces are and will be defending our country's land, air, and sea borders.”

Trump had warned the previous day that he had information that “Iran or its proxies are planning a sneak attack on U.S. troops and/or assets in Iraq,” adding that

Baqeri, however, claimed that attacks on U.S. troops were only “the natural reaction of the Iraqi people and their resistance to the evil act of assassinating General Qasem Soleimani and Abu Mahdi al-Muhandis.”

U.S. forces killed Soleimani, head of Iran’s clandestine operations in the region, alongside Iraqi militia kingpin al-Muhandis in a sneak attack outside Baghdad International Airport on January 3.

The killing followed a week of tensions at the end of last December. Rockets had killed an American translator in Iraq, and the U.S. military pointed the finger at a militia under Soleimani and al-Muhandis’s control. U.S. forces retaliated to the attack by killing 25 members of the militia, which then ransacked the U.S. Embassy in Baghdad on New Years’ Eve.

Iran responded to Soleimani’s killing with a ballistic missile strike on a U.S. base in Iraq that injured more than a hundred American personnel on January 7.

Tensions have ratcheted up again after a missile attack on March 11 killed two Americans and a British medic in Iraq. U.S. forces responded by pummeling Iranian-backed militia positions, but the assailants struck the same base again on March 14, wounding five more Americans.

A new group calling itself the “League of Revolutionaries” took responsibility for the attacks, although a well-known Iranian-backed militia called Kata’ib Hezbollah also gloated that it was protecting the attackers.

Members of the Trump administration have been debating the proper response to these attacks. National Security Advisor Robert O’Brien and Secretary of State Mike Pompeo reportedly argued that the coronavirus crisis would leave Iran in a weak position, while Lt. Gen. Robert P. White warned in a leaked memo that a military escalation would be dangerous.

U.S. forces are now withdrawing from several smaller bases across Iraq. The U.S.-led coalition denies that the movement is related to Iranian threats.

“As a result of the success of [Iraqi Security Forces] in their fight against ISIS, the Coalition is repositioning troops from a few smaller bases,” a Coalition spokesperson said in a March 20 statement. “These bases remain under Iraqi control and we will continue our advising partnership for the permanent defeat of [ISIS] from other Iraqi military bases, providing much-needed specialist support.”

Matthew Petti is a national security reporter at the National Interest. Follow him on Twitter: @matthew_petti.