WASHINGTON (Reuters) - The White House sent to Congress on Saturday formal notification of Friday’s U.S. drone strike that killed a top Iranian general, amid complaints from Democrats that President Donald Trump did not notify lawmakers or seek advance approval for the attack.

Iranians gather to mourn General Qassem Soleimani, head of the elite Quds Force, who was killed in an air strike at Baghdad airport, in Tehran, Iran January 4, 2020. Nazanin Tabatabaee/WANA (West Asia News Agency) via REUTERS

The U.S. strike that killed Iranian military commander Qassem Soleimani in Iraq has caused a dramatic rise in tensions between Iran and the United States and its allies.

The classified notification was sent under a 1973 U.S. law called the War Powers Act, which requires the administration to notify Congress within 48 hours of committing armed forces to military action or imminent actions.

White House national security adviser Robert O’Brien confirmed the notice had been sent and told Fox News on Saturday there were numerous legal grounds for the attack. He said Justice Department lawyers had signed off on the plan.

“This was an absolute legal operation,” O’Brien said. “We feel very good about the legal basis for this military operation.”

House of Representatives Speaker Nancy Pelosi, a Democrat, said it raised more questions than it answered.

“This document prompts serious and urgent questions about the timing, manner and justification of the Administration’s decision to engage in hostilities against Iran,” Pelosi said in a statement.

She said the “highly unusual” decision to classify the entire document compounded her concerns and “suggests that the Congress and the American people are being left in the dark about our national security.”

A senior Democratic aide described the notification as “brief and insufficient.”

In the notification, Trump’s administration was expected to have explained the circumstances, the authority under which the action was taken and the expected scope and duration of the military involvement. The White House declined to comment.

Senator Bernie Sanders, a contender for the Democratic nomination to run against Trump in November’s presidential election, said on Saturday that Congress must take immediate steps to restrain the president “from plunging our nation into yet another endless war.”

Senator Elizabeth Warren, another Democratic presidential candidate, told reporters on Saturday that Trump has taken the United States “to the brink of war” and called his actions “reckless. He evidently has no plan.”

Pelosi has said the strike in Baghdad was conducted without specific authorization from Congress and “without the consultation of the Congress.”

On Saturday she described the administration’s military engagement as “provocative, escalatory and disproportionate” and said it put U.S. citizens, troops and their allies in danger.

The Pentagon on Friday briefed staff members from the House Armed Services Committee and Senate Armed Services Committee on the attack.

On Friday, Democratic U.S. Senator Tim Kaine introduced a resolution to force a debate and vote in Congress to prevent further escalation of hostilities with Iran.

Republicans in Congress have generally backed Trump’s action. Senate Majority Leader Mitch McConnell said on Friday that “no man alive was more directly responsible for the deaths of more American service members than Qassem Soleimani.”