U.S. officials have refused to confirm the number of Americans still captive or unaccounted for because they say that might compromise their safety.
Still, the U.S. flatly rejected an offer by the militants — led by a Mali-based al-Qaida offshoot known as the Masked Brigade — to free two American hostages in exchange for the release of Omar Abdel Rahman, a blind sheikh convicted of plotting to blow up New York City landmarks and considered the spiritual leader of the 1993 World Trade Center bombing, and Aafia Siddiqui, a Pakistani scientist convicted of shooting at two U.S. soldiers in Afghanistan. Both are jailed in the United States.
"The United States does not negotiate with terrorists," State Department spokeswoman Victoria Nuland said.
The desert siege began Wednesday when the militants attempted to hijack two buses at the plant, were repelled, and then seized the gas refinery. They said the attack was retaliation for France's recent military intervention against Islamist rebels in neighboring Mali, but security experts argue it must have taken weeks of planning to hit the remote site.
Since then, Algeria's government has kept a tight grip on information about the siege.
Clinton stressed that American officials would stay in close contact with their Algerian counterparts. Sellal, she said, made clear that the Algerian operation against the militants "was still ongoing, that the situation remained fluid, that the hostages remain in danger in a number of instances."
Speaking beside Japan's new foreign minister, Fumio Kishida, Clinton said the crisis underscored the threat posed by extremists in North Africa, where al-Qaida-linked militants have seized control of half of Mali and plunged the country into civil war. She vowed to enhance U.S. work with Algeria and other countries in the region to combat terrorists even after the hostage situation ends.
"It is absolutely essential that as we work to resolve this particular terrible situation, we continue to broaden and deepen our counterterrorism cooperation," Clinton said. "We will not rest until we do as much as we can ... to restore security to this vital region, and to bring those who would terrorize and kill innocent people to justice."
Associated Press writers Lolita C. Baldor in London, Robert Burns in Washington and Ramit Plushnick-Masti in Houston contributed to this report.