A day after US Secretary of State Hillary Clinton warned that the Pakistani government was ceding ground to the Taliban, a top Israeli defense official expressed concern on Thursday that the country's nuclear arsenal would fall into extremist hands and be used to threaten Israel. Pakistan is believed to have several dozen nuclear warheads. On Wednesday, in testimony before the House Foreign Affairs Committee, Clinton said the Pakistani government was losing control of the country. "I think we cannot underscore [enough] the seriousness of the existential threat posed to the state of Pakistan by the continuing [Islamist] advances," Clinton said, adding that an unstable and nuclear-armed Pakistan would pose a "mortal threat" to the United States and other countries. Israeli defense officials said that while Pakistan was farther away than Iran, Israel needed to be concerned with the political upset there. "This is a major challenge," one top official told The Jerusalem Post. "If Pakistan's nuclear weapons falls into hands of radical Islamic elements, this will be a nightmare for the West and will also affect us," he said. The warnings by Israeli officials came as Pakistani Islamists expanded into new territory adjacent to the Swat Valley, where they intend to impose Shari'a law. US President Barack Obama has been trying to implement a new stabilization strategy for the region and Adm. Michael Mullen, chairman of the Joint Chiefs of Staff, traveled to Pakistan earlier this week for meetings with government officials.