New York City public health officials are warning providers that a “serious shortage” of swabs used to perform coronavirus tests could cause some hospitals to run out.

In a memo to providers, the New York City Department of Health and Mental Hygiene reiterated that only hospitalized patients with symptoms should be testing for COVID-19 to preserve resources and swabs for patients with severe illness.

The department didn’t state in Saturday’s memo what the state would do to procure more swabs, but other health departments have also warned that shortages of these supplies could hamper COVID-19 testing in the U.S.

The pandemic has strained the supply chain for these items as the U.S. competes with other countries battling the novel coronavirus.

“As the swab supply continues to decline, there is a real possibility hospitals will completely run out,” the department wrote in the memo.

States are also running low on chemicals that are needed to process the tests and personal protective equipment (PPE) used to shield workers from the coronavirus.

PPE is also needed by lab workers to ensure they aren’t exposed to the virus.

New York City health officials also noted in the memo that its supply of PPE is “tenuous.”

New York Gov. Andrew Cuomo (D) has repeatedly said wide-spread testing is needed to reopen the U.S. economy, but he is referring to antibody tests that don’t require the use of swabs and can identify whether someone has already had COVID-19.

Still, those tests, which test blood instead of a patient’s upper respiratory fluid, are not yet widely available and questions have been raised about their accuracy.