Negotiations on a multibillion-dollar disaster aid bill in the Senate have grown more complicated in recent days with a push by the Trump administration to add emergency spending for the U.S.-Mexico border to the bill, lawmakers and others involved in the talks said Tuesday.

Senate Appropriations Committee Chairman Richard C. Shelby (R-Ala.) personally made the point to Vice President Pence on Tuesday that introducing a new element into the disaster aid talks could only complicate matters, according to a person familiar with their conversation who spoke on the condition of anonymity to discuss it.

But with the $17 billion disaster aid bill among the few legislative vehicles expected to move through Congress anytime soon, administration officials have begun eyeing it to carry some or all of a separate $4.5 billion emergency spending request for the border they sent to Congress last week. Their effort has met resistance from lawmakers of both parties and led to even greater uncertainty over a path forward for the disaster bill, which includes money for multiple U.S. states and territories but already has been delayed for months.

The talks over the disaster aid bill were already fraught because of a fight between President Trump and congressional Democrats over spending for Puerto Rico. Trump has yet to agree to a deal offering more than $600 million for the U.S. island territory’s food stamp program, while Democrats say much more is required for needs on many fronts as Puerto Rico recovers slowly from the devastations of Hurricane Maria. Senate Republicans have been prepared to offer more — though perhaps not enough to satisfy Democrats — but it has been unclear whether the White House would support their plans.

On Tuesday, Pence joined Senate Republicans for a closed-door lunch where he encouraged their efforts to find a deal on the disaster bill, telling them: “ ‘Do your best,’ ” according to Sen. Kevin Cramer (R-N.D.).

At the same time, Pence reiterated Trump’s views on Puerto Rico, displaying a graph aimed at bolstering Trump’s claims that Puerto Rico has already been given $91 billion, much more than many states have received for their own hurricanes and other natural disasters. However, as documented by fact-checkers at The Washington Post, the $91 billion figure represents an estimate of potential liabilities on the island over coming years, while a much smaller sum — $40.8 billion — has been allocated to Puerto Rico. Even less than that has actually been spent.

Pence also suggested to senators that a study be conducted to track spending on Puerto Rico.

“I think what the vice president was saying is that we ought to understand how we spend money like this,” said Sen. Rick Scott (R-Fla.). “I’m very supportive of Puerto Rico. We have to have accountability, which I believe in.”

Senators left the lunch uncertain how or when a deal would be reached on the long-stalled disaster aid bill. The hang-up is causing intense frustration among many GOP senators, particularly those from the South, where states were battered by hurricanes and tornadoes, and the Midwest, which has recently been ravaged by floods.

The Democratic-led House plans to pass its own version of the disaster aid legislation Friday, with much more generous spending for Puerto Rico, but that appears unlikely to break the logjam in the Senate. Shelby said a deal was unlikely to be reached this week.

Shelby said he told Pence during the lunch, “We think we’re close to a resolution and then we’re far apart, and there are more obstacles put in, either by one party or the other. And I said in there, ‘This is important. Disaster is important to Democrats, it’s important to Republicans, it’s important to everybody that’s got a disaster situation, and that’s a lot of states.’

“And I said, ‘This is the longest I remember I’ve ever seen something this important not be resolved.’ ”

Shelby also pointed out to Pence, and to acting White House chief of staff Mick Mulvaney in an earlier phone call, that as important as the disaster aid bill is, budget fights that loom later this year are even more consequential. And the failure to reach swift resolution on the disaster bill bodes poorly for those other issues, Shelby said, with “draconian” budget caps set to snap into place absent a spending deal, and the potential for another government shutdown Oct. 1.

Democrats criticized the administration’s behind-the-scenes efforts to attach its border spending request to the disaster aid bill. The border request is largely for humanitarian purposes to address the tide of Central American migrants arriving at the border, but it includes elements opposed by Democrats who say the result could be longer undue detentions of peaceful migrants.

“Disaster victims have been waiting far too long for the White House and Senate Republicans to get their act together,” said Evan Hollander, a spokesman for House Appropriations Committee Chairwoman Nita M. Lowey (D-N.Y.). “There is no reason that disaster relief should be contingent on unrelated immigration spending.”

Republican senators left the lunch meeting with Pence saying that the administration supported reaching some deal on disaster but unsure exactly what they would support.

“He’s concerned about Puerto Rico — we all are. But we also know that in a divided government, we have to give to get,” Cramer said. “So his basic message was, ‘Do your best, do your best, we have to get something done.’ ”

Pence’s chief of staff, Marc Short, said the vice president conveyed Trump’s desire for a deal, but also his concerns about Puerto Rico.

“The vice president was making the point that the president recognizes people in some of these areas are hurting and that he wants them to get relief,” Short said. “But he thinks the money going to Puerto Rico is excessive and is resentful that this is being tied up over that.”