President Donald Trump speaks to supporters where he formally announced his 2020 re-election bid, June 18, 2019, in Orlando, Fla.
President Donald Trump speaks to supporters where he formally announced his 2020 re-election bid, June 18, 2019, in Orlando, Florida.

WASHINGTON - VOA's Carla Babb, Steve Herman, Katherine Gypson, Ken Bredemeier contributed to this report

President Donald Trump has fired a warning shot at Iran via social media, hours after U.S. defense officials confirmed Tehran shot down a U.S. drone over international airspace.

“Iran made a very big mistake!” Trump said on Twitter Thursday, as tensions between Washington and Iran continued to escalate following the alleged attacks on two oil tankers in the Strait of Hormuz last week.

Earlier Thursday, Iran’s Revolutionary Guard claimed to have shot down a U.S. RQ-4 Global Hawk drone over Iranian airspace.

“Borders are our red line,” Revolutionary Guard commander Gen. Hossein Salami told a crowd in the western city of Sanandaj, in a televised address. “`Any enemy that violates the borders will be annihilated.”

But U.S. Central Command, which oversees U.S. military activity in the region, quickly refuted the Iranian claim.

“This was an unprovoked attack on a U.S. surveillance asset in international airspace,” CENTCOM spokesman Capt. Bill Urban said in a statement, arguing the drone had been operating over the Strait of Hormuz when it was taken down by a surface-to-air missile on Wednesday.

The aircraft, costing more than $222 million, can surveil about 100,000 kilometers squared a day, which is about the size of South Korea or Iceland. The U.S. said it had dispatched naval ships to the strait in an attempt to recover pieces of the drone from a debris field.

Concern about a potential confrontation between the U.S. and Iran has been growing since U.S. officials last week accused Iran of being responsible for mine attacks on two oil tankers in the Strait of Hormuz.

In addition, U.S. officials have said intelligence over the past month and a half has pointed to numerous “threat streams” coming out of the Middle East that “link back to the Iranian regime.”

WATCH: US Navy video of attack on drone

On Wednesday, U.S. Special Representative for Iran, Brian Hook, told a panel of lawmakers that the decision to authorize another 1,000 troops to bolster defenses at U.S. positions in Iraq and Syria – including a Patriot missile battery and additional manned and unmanned reconnaissance aircraft -- had been effective.

“We think that has helped to decrease the risk of miscalculation," he said. “What we’ve seen so far has not been on the scale that we have expected.”

But following the downing of the U.S. drone, South Carolina Republican Senator Lindsey Graham said the risks of a possible confrontation are rising.

"He  believes that we’re getting into a bad space, that his options are running out," Graham told reporters after speaking with President Trump.

House Speaker Nancy Pelosi said Thursday the United States has "no appetite" for war with Iran.

"It's a dangerous situation, high tension wires are up in the region. We have to be strong and strategic about how we protect our interests. We also cannot be reckless in what we do," the top Democratic lawmaker told reporters before heading to an intelligence briefing.

Russian President Vladimir Putin warned the U.S. against retaliating for the incident, saying U.S. use of force against Iran "would be a disaster for the region."

The U.S. military said Iran first tried to shoot down a U.S. drone last week.

Relations have deteriorated since Trump withdrew last year from the international agreement that limited Iran's nuclear program in exchange for sanctions relief.

Since then, the U.S. has levied a variety of sanctions against Iran as part of what the a “maximum pressure” campaign against Iran, aimed at bringing Tehran back to negotiations in order to secure what White House officials have described as a more comprehensive nuclear deal that also addresses Iran’s missile program and its support for terrorist groups.

U.S. officials believe the Iranian government is feeling the pressure from the renewed sanctions.  But in response, Iran announced increased production of low-enriched uranium as it seeks help from European nations to circumvent the U.S. measures.

Following the attacks on the Norwegian and Japanese oil tankers last week, senior U.S. military officials said the risk for miscalculation was growing.

Iran has denied responsibility for the attacks on the tankers.