Iranian Foreign Minister Mohammad Javad Zarif said his country "only acts in self-defense" after U.S. President Donald Trump warned Tehran of a "heavy price" if it or its allies attack U.S. troops or assets in Iraq.
The war of words comes amid renewed tensions between the two arch-foes despite the coronavirus pandemic.
"Upon information and belief, Iran or its proxies are planning a sneak attack on U.S. troops and/or assets in Iraq," Trump tweeted on April 1.
"If this happens, Iran will pay a very heavy price, indeed!" he added.
In response, Zarif wrote in a tweet on April 2 that "Iran has FRIENDS: No one can have MILLIONS of 'proxies.'"
"Unlike the U.S. -- which surreptitiously lies, cheats & assassinates -- Iran only acts in self-defense," he added.
Iran and the United States are in a tense battle for influence in Iraq, where Tehran has powerful allies and Washington has close ties to the government.
Over the past year, Washington has accused Iranian-backed militias of attacks on Iraqi military bases hosting coalition forces and on foreign embassies, particularly the U.S. mission.
Tensions between Tehran and Washington have risen sharply since Trump in 2018 withdrew the United States from a landmark 2015 nuclear agreement between Iran and world powers and reimposed sweeping sanctions on the Islamic Republic.
Tehran has repeatedly called on Washington to lift the sanctions, particularly since the start of the coronavirus epidemic.
Medicines and medical equipment are technically exempt from the U.S. sanctions but purchases are frequently blocked by the unwillingness of banks to process transactions for fear of incurring large penalties in the United States.
Iran is one of the countries worst hit by the coronavirus, which has infected 50,468 people in the country, according to the Health Ministry.
Ministry spokesman Kianush Jahanpur on April 2 announced 124 more fatalities, raising the death toll to 3,136.
Meanwhile, parliamentary speaker Ali Larijani tested positive for coronavirus and is receiving treatment in quarantine. Larijani is the latest high-ranking Iranian official to test positive for the COVID-19 illness caused by the virus.
Experts have been skeptical about the veracity of official figures released by the Iranian authorities, who keep a tight lid on local and foreign media.
The pandemic has also hit the United States hard, with almost 217,000 infections and more than 5,100 deaths.