NEW YORK/WASHINGTON (Reuters) – Hundreds of flights were grounded or delayed at three New York-area and Philadelphia airports as more air traffic controllers called in sick on Friday in one of the most tangible signs yet of disruption caused by a 35-day partial federal government shutdown.

Hundreds of thousands of federal workers have been furloughed or, as with some airport workers, required to work without pay. Some federal agencies have reported much higher absence rates among workers as they face an indefinite wait for their next paychecks.

The Federal Aviation Administration briefly issued a ground stop for arriving flights at New York’s LaGuardia Airport on Friday before lifting it about an hour later. Staff shortages also delayed flights at Newark Liberty International Airport and Philadelphia International Airport, the FAA said.

The delays immediately became a new flashpoint in the political standoff between the Democrat-controlled U.S. House of Representatives and U.S. President Donald Trump over the shutdown, caused by a dispute about funding for Trump’s plan to increase barriers on the U.S.-Mexico border.

Democrats in the House are demanding a reopening of the government before any negotiations with Trump and his Republican allies in Congress on border security.

The disruptions come the day after the U.S. Senate rejected two shutdown-ending bills as hundreds of thousands of federal workers missed a second paycheck on Friday.