U.S. Republican Sen. Lisa Murkowski threatened Friday to invoke congressional powers to stop U.S. President Donald Trump from following through on his threats to seize Greenland.

Speaking to reporters in Copenhagen after taking part in a bipartisan delegation of U.S. lawmakers meeting with Danish and Greenlandic officials, Murkowski — an Alaskan who is a regular critic of the president — said it was “an important message for the people of the Kingdom of Denmark to understand” that the U.S. has three branches of government.

“In Congress, we have tools at our disposal under our constitutional authority that speaks specifically to the power of the purse through appropriations,” she said, referring to congressional control of federal spending. She added that Greenland, a self-ruling Danish territory, should be seen as an “ally” rather than an “asset.”

Democratic Sen. Chris Coons of Delaware, who also took part in the visit by House and Senate lawmakers, said he would push ahead with legislation to curb Trump’s power to act unilaterally.

A bipartisan group of American lawmakers introduced a bill this week to prevent Washington from invading a fellow NATO member. (Greenland, as a Danish territory, is part of the Atlantic alliance.) Congress can force votes on constraining presidential war powers, but recent efforts to rein in Trump have not succeeded. Even if it did pass, the White House has asserted any such measure would be unconstitutional.

Coons also undercut Trump’s arguments about Greenland from a national security perspective.

“Are there real, pressing threats to the security of Greenland from China and Russia?” Coons said. “No, not today.”

Trump has invoked the specter of Russian and Chinese warships in the Arctic as an argument for seizing control of Greenland. Conns said such claims were “rhetoric” rather than “reality.”

The president’s threats have sparked a full-blown diplomatic transatlantic crisis. Danish Foreign Minister Lars Løkke Rasmussen and his Greenlandic counterpart Vivian Motzfeldt met with U.S. Vice President JD Vance and Secretary of State Marco Rubio earlier this week at the White House to discuss the matter as European nations rushed to deploy troops to the Arctic territory.

White House press secretary Karoline Leavitt said after the Vance-Rubio summit that the president was still focused on acquiring Greenland.

Trump warned in remarks Friday he “may put a tariff on countries if they don’t go along with Greenland.”

The Kremlin’s chief spokesperson Dmitry Peskov said Friday the situation in Greenland was highly unusual “from the point of view of international law,” adding Moscow would “watch together with the whole world” as events unfold.