United States President Donald Trump said on Thursday that “Iran made a very big mistake” in shooting down an American spy drone near the Strait of Hormuz.

Earlier in the day, the United States accepted claims by Iran’s Islamic Revolutionary Guard Corps that it brought down the surveillance drone. However, the US said that the drone was shot down not over Iranian airspace but in international waters.

However, Iranian Foreign Minister Mohammad Javad Zarif rubbished US claims. “The US wages #EconomicTerrorism on Iran, has conducted covert action against us & now encroaches on our territory,” he tweeted. “We don’t seek war, but will zealously defend our skies, land & waters. We’ll take this new aggression to #UN & show that the US is lying about international waters.”

Meanwhile, Russian President Vladimir Putin warned that any use of force by the US against Iran would lead to disaster, AFP reported. “The US says it does not rule out the use of force... This would be a disaster for the region,” Putin said in Moscow. “It would lead to a surge in violence and an increase in the number of refugees.”

Putin also said that US sanctions against Iran are unreasonable.

Iran’s Revolutionary Guard Corps on Thursday said the US-made Global Hawk surveillance drone was brought down by its Air Force near the Kouh-e Mobarak region in the central district of Jask County. However, US Central Command spokesperson Bill Urban had earlier in the day said “no US aircraft were operating in Iranian airspace”.

The incident comes at a time when the United States has accused Iran of attacking two of its oil tankers in the Gulf of Oman on June 13. Iran has repeatedly warned it might block the Hormuz Strait in retaliation and denied its involvement in the attacks.

The tensions come in the backdrop of an escalating standoff between the US and Iran one year after Donald Trump withdrew from the 2015 Iran nuclear deal, and reimposed heavy sanctions on the country. Iran responded to the sanctions by threatening to walk away from its obligations under the nuclear deal – which had promised economic relief in exchange for limits to its nuclear development – and return to higher levels of uranium enrichment.