WASHINGTON — National security adviser Jake Sullivan is set to meet Tuesday morning with hostage families of Americans believed to be held by Hamas in Gaza, according to a senior administration official. 

The White House meeting comes at the most critical juncture yet of the eight-month conflict and fragile negotiations for a possible cease-fire deal that would include the release of remaining hostages over several phases, which President Joe Biden outlined in a surprise address Friday. 

Sullivan is expected to deliver a “regular update” following Biden’s speech Friday, and all families are expected to be represented in person or on the phone, according to details first shared with NBC News. 

It will be Sullivan’s ninth meeting with the families since Oct. 7, when Hamas terrorists killed 1,200 people in Israel and kidnapped more than 250 others. 

Biden has urged Israel and Hamas to accept the latest proposal, stressing Friday that he believes it is “time for the war to end.” But it’s unclear whether Hamas will accept the deal, and Israel and the U.S. have traded different versions about what they believe the agreement entails. 

Throughout the war, senior White House officials have regularly stayed in touch with the hostages’ families. Four Americans have been released since the war started, and the Biden administration believes Hamas still holds five American citizens. 

Hamas is believed to have killed several U.S. citizens on Oct. 7, with their bodies then taken into Gaza. Their families are pleading for the remains of their loved ones to be released. 

Negotiations for a hostage deal have faced many hurdles in recent months. Talks most recently stalled last month before this latest round of talks. 

Sullivan met last week with 17 diplomats from countries whose citizens Hamas is holding in Gaza, a U.S. official said. The U.S. has coordinated a joint statement from all of the countries calling for the hostages to be freed, which was released in April. 

The involved countries are: Argentina, Austria, Brazil, Bulgaria, Canada, Colombia, Denmark, France, Germany, Hungary, Poland, Portugal, Romania, Serbia, Spain, Thailand and the United Kingdom.