Still, the new Pentagon total would suggest that at least eight of the countries who have signed up have also declined to be publicly named, in a sign of political sensitivities of the operation.
"We've had over 20 nations now sign on to participate," Major General Patrick Ryder said on Thursday, noting declarations by Greece and Australia.
"We'll allow other countries, defer to them to talk about their participation," he added.
US Defense Secretary Lloyd Austin officially announced the creation of the international force, Operation Prosperity Guardian, earlier this week.
In the initial announcement, it was stated that the new mission consists of the US, France, the United Kingdom, Bahrain, Canada, Italy, Netherlands, Norway, Seychelles and Spain.
The Iranian-backed Houthi rebels have upped their attacks on vessels in the Red Sea since the start of the Israel-Hamas was on October 7.
Recently, the Houthi rebels threatened to attack any vessels heading to Israeli ports unless food and medicine were allowed into the Gaza Strip.
Two days later, the Houthis launched a land-based cruise missile towards a Norwegian commercial tanker vessel.
A senior Houthi official then warned cargo ships in the Red Sea to avoid traveling toward “occupied Palestinian territories”.
Several shipping companies have announced in recent days that they would temporarily halt operations in the Red Sea after their vessels came under attack from the Houthis.