It will be the "official end of Iran" if the country threatens the United States again, U.S. President Donald Trump tweeted Sunday, as the increasingly strained relationship between Washington and Tehran raises concerns about a potential U.S.-Iran conflict.

If Iran wants to fight, that will be the official end of Iran. Never threaten the United States again! —@realDonaldTrump

Politicians in Canada and the United States have been expected to turn their attention to Iran next week amid heightened concerns over the Trump administration's sudden moves in the region.

Trump has tightened economic sanctions against Iran, and his administration says it has built up the U.S. military presence in the region. It accuses Iran of threats to U.S. troops and interests.

Tehran has described U.S. moves as "psychological warfare" and a "political game," while Iranian Foreign Minister Mohammad Javad Zarif posted his own message Monday on Twitter, saying Trump had been "goaded" into "genocidal taunts."

Goaded by <a href="https://twitter.com/hashtag/B_Team?src=hash&ref_src=twsrc%5Etfw">#B_Team</a>, <a href="https://twitter.com/realDonaldTrump?ref_src=twsrc%5Etfw">@realdonaldTrump</a> hopes to achieve what Alexander, Genghis & other aggressors failed to do. Iranians have stood tall for millennia while aggressors all gone. <a href="https://twitter.com/hashtag/EconomicTerrorism?src=hash&ref_src=twsrc%5Etfw">#EconomicTerrorism</a> & genocidal taunts won't "end Iran". <a href="https://twitter.com/hashtag/NeverThreatenAnIranian?src=hash&ref_src=twsrc%5Etfw">#NeverThreatenAnIranian</a>. Try respect—it works! —@JZarif

U.S. House Democrats have invited former CIA director John Brennan to speak about the situation in Iran next week.

Brennan, an outspoken critic of President Donald Trump, is scheduled to talk to House Democrats at a private weekly caucus meeting Tuesday, according to a Democratic aide and another person familiar with the private meeting. Both were granted anonymity to discuss the meeting.

The invitation to Brennan and Wendy Sherman, a former State Department official and top negotiator of the Iran nuclear deal, offers counter-programming to the Trump administration's closed-door briefing for lawmakers also planned for Tuesday on Capitol Hill. Democratic lawmakers are likely to attend both sessions.

Withdrawing personnel from Iraq

The Trump administration recently sent an aircraft carrier and other military resources to the Persian Gulf region, and withdrew nonessential personnel from neighbouring Iraq, raising alarm among Democrats and some Republicans on Capitol Hill over the possibility of a confrontation with Iran.

Pedestrians and drivers make their way through a street in downtown Tehran on Saturday. Politicians in Canada and the United States are expected to turn their attention to heightened concerns over Iran and the Trump administration in the coming week. (Vahid Salemi/Associated Press)

There are questions about what prompted the actions, and many lawmakers have demanded more information.

Canada's Department of Global Affairs released a statement earlier Sunday saying its representatives "continue to work closely with our allies and partners to hold Iran to account."

Canada 'concerned'

"Canada is concerned by escalating tensions in the region, and supports diplomatic and constructive approaches to address the situation," a spokesperson said by email.

"We oppose Iran's support for terrorist organizations, its threats toward Israel, its ballistic missile program, its ongoing human rights violations, and its support for the murderous Assad regime. We remain concerned by Iran's nuclear program, and call on Iran to continue to uphold its commitments under the Joint Comprehensive Plan of Action."

Trump and Brennan have clashed openly, particularly over the issues surrounding the special counsel's probe of Russian interference in the 2016 election. Brennan stepped down from the CIA in 2017.

The president last year said he was revoking the former spy chief's security credentials after Brennan was critical of Trump's interactions with Russian President Vladimir Putin at a summit in Helsinki. Top national security officials often retain their clearance after they have left an agency as a way to provide counsel to their successors. It's unclear if Brennan actually lost his clearance.

House Speaker Nancy Pelosi had been asking the administration for a briefing for all lawmakers on the situation in Iran, but she said the request was initially rebuffed. The administration provided a classified briefing for leaders of both parties last week.

Rep. Tulsi Gabbard, one of 24 Democrats vying for a presidential nomination, said U.S. President Donald Trump is 'leading us down this dangerous path towards a war with Iran.' (Joshua Roberts/Reuters)

Trump has said he is not pushing for war with Iran. During the 2016 presidential campaign, he promised to stay out of overseas conflicts, saying the wars in Afghanistan and Iraq were too costly.

Rep. Tulsi Gabbard, one of 24 Democrats vying for a presidential nomination, said on ABC's This Week with George Stephanopoulos that Trump was "leading us down this dangerous path toward a war with Iran."

"He says he doesn't want it, but the actions of him and his administration, people like John Bolton and Mike Pompeo, tell us a very different story," she said.

"They are setting the stage for a war with Iran that would prove to be far more costly, far more devastating and dangerous, than anything that we saw in the Iraq war," Gabbard said.

Rocket fired into Iraq's Green Zone

A rocket was fired into Iraqi capital Baghdad's heavily fortified Green Zone, which houses government buildings and foreign embassies, but caused no casualties, the Iraqi military said on Sunday.

The embassy in Baghdad and the U.S. Consulate in the Iraqi Kurdish regional capital Erbil had moved out non-emergency staff this week out of apparent concern about perceived threats from Iran.