Five GOP senators are pressing Secretary of State Rex Tillerson Rex Wayne TillersonGary Cohn: 'I haven't made up my mind' on vote for president in November Kushner says 'Alice in Wonderland' describes Trump presidency: Woodward book Conspicuous by their absence from the Republican Convention MORE to expel all Cuban diplomats from the U.S. and shutter the American Embassy in Havana.

In a letter to Tillerson dated Thursday, Republican Sens. Tom Cotton Tom Bryant CottonRenewed focus on Trump's Supreme Court list after Ginsburg's death Republicans call for DOJ to prosecute Netflix executives for releasing 'Cuties' Loeffler calls for hearing in wake of Netflix's 'Cuties' MORE (Ark.), Richard Burr Richard Mauze BurrRep. Mark Walker says he's been contacted about Liberty University vacancy Overnight Defense: Trump rejects major cut to military health care | Senate report says Trump campaign's Russia contacts posed 'grave' threat Senate report describes closer ties between 2016 Trump campaign, Russia MORE (N.C.), John Cornyn John CornynCalls grow for Biden to expand election map in final sprint Bipartisan praise pours in after Ginsburg's death Chamber of Commerce endorses McSally for reelection MORE (Texas), Marco Rubio Marco Antonio RubioSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Florida senators pushing to keep Daylight Savings Time during pandemic Hillicon Valley: DOJ indicts Chinese, Malaysian hackers accused of targeting over 100 organizations | GOP senators raise concerns over Oracle-TikTok deal | QAnon awareness jumps in new poll MORE (Fla.) and James Lankford James Paul LankfordMcConnell works to lock down GOP votes for coronavirus bill Charities scramble to plug revenue holes during pandemic Warren calls for Postal Service board members to fire DeJoy or resign MORE (Okla.) admonished Cuba for failing to ensure the security of U.S. diplomats based in the country.

"Cuba’s neglect of its duty to protect our diplomats and their families cannot go unchallenged," the letter reads.

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The letter comes as the number of Americans confirmed to have suffered mysterious health symptoms during stints in Havana continues to rise. That number was updated to 21 this week.

Some Americans have experienced permanent hearing loss and mild brain damage, or concussions, from the so-called "health attacks," the cause of which remains unknown.

"The safety of U.S. diplomatic personnel and their families posted overseas remains one of our high priorities and a shared responsibility of those nations that host U.S. diplomatic facilities," the senators wrote. "We urge you to remind the Cuban government of its obligation and to demand that it take verifiable action to remove these threats to our personnel and their families."

Symptoms were first reported in fall 2016 and were initially believed to have continued until spring 2017. But the State Department revealed earlier this month that an incident happened as recently as August. U.S. officials are investigating the matter.

The incidents were made public in early August, when the State Department revealed that the U.S. had expelled two Cuban diplomats from the country's embassy in Washington.

It remains unclear who is responsible for the attacks, and the Cuban government has repeatedly denied any wrong-doing. The Associated Press reported Friday that, after hearing of the attacks, Cuban President Raul Castro voiced concern and befuddlement to Jeffrey DeLaurentis, a U.S. diplomat in Havana.

The Cuban government even offered to allow the FBI to investigate the matter — a level of access rarely offered by foreign governments, according to the AP.

The U.S. relationship with Cuba is a tense one, after President Trump earlier this year said his administration would clamp down on the travel and trade restrictions former President Obama had relaxed during his tenure.