With questions mounting over President Donald Trump's tough talk on Iran, top national security officials are heading to Capitol Hill to brief Congress. But skeptical Democrats have asked for a second opinion.

The competing closed-door sessions Tuesday will be unusual and potentially polarizing. They come after weeks of escalating tensions in the Persian Gulf that have raised alarms over a possible military confrontation with Iran. Lawmakers are warning the Trump administration that it can't take the country into war without approval from Congress.

Trump, veering between bombast and conciliation in his quest to contain Iran, threatened Monday to meet provocations by Iran with "great force." But he also said he's willing to negotiate.

"We'll see what happens," Trump told reporters Monday.