U.S. Senate Republicans have blocked a resolution that would have barred President Donald Trump from launching military action against Venezuela without congressional approval, despite growing concern over recent U.S. strikes in the southern Caribbean.

The Senate voted 51 to 49, largely along party lines, against moving forward with the war powers resolution.

Only two Republicans broke ranks to join Democrats in supporting the measure, the latest test of the party’s willingness to back Trump’s expanding military operations near Venezuela.

According to administration officials, U.S. forces have carried out at least 16 strikes against vessels in the Pacific and southern Caribbean since early September, killing more than 65 people.

The surge in attacks has raised fears that Trump may authorise a direct strike on Venezuelan territory — a concern that led to the bipartisan resolution, co-sponsored by Democrats Tim Kaine and Adam Schiff and Republican Rand Paul.

Officials told lawmakers this week that Washington currently has no plans to target Venezuelan soil.

Members of Congress from both parties had complained they have received scant information, such as who was killed, evidence of drug trafficking, the buildup's cost or the administration's long-term Latin American strategy.

During debate on the resolution on Thursday, Senate Democratic leader Chuck Schumer of New York called on the Trump administration to hold a briefing for all 100 senators. Democrats have also called for a meeting with the full House of Representatives.

The administration has said those targeted were "narco-terrorists" transporting drugs that endangered Americans, without providing evidence or publicly explaining the legal justification for attacking the boats rather than stopping them and arresting those on board.

The U.S. Constitution requires any president to obtain Congress' approval before launching a prolonged military operation.

"The time is right for Congress to step in and reassert our congressional responsibility," Schiff told a news conference before Thursday's vote.

Opposing the resolution, Senator Jim Risch of Idaho, the Republican chairman of the Senate Foreign Relations Committee, said Trump had the authority he needed, as commander-in-chief.

"I wish my colleagues would join me today in congratulating the president for what he has done," he said.

It was only the latest attempt to rein in Trump's war powers. The Senate blocked a resolution last month, by a vote of 51-48, that sought to stop the boat strikes. That vote was also mostly along party lines with the same two Republicans - Paul and Lisa Murkowski of Alaska - backing the measure.

Kaine and Schiff told reporters on Thursday they would consider options for trying another resolution after the vote.

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