The United States will maintain a "credible deterrent" for friends and allies even though the administration is looking for additional ways to move President Barack Obama's vision for a nuclear-free world forward, the State Department said Tuesday.

The Obama administration is reportedly considering renouncing the preemptive nuclear strike option to bolster his legacy as champion of a world without nuclear weapons. The so-called "no first use" policy has unnerved allies depending on U.S. nuclear weapons for their security.

State Department deputy spokesman Mark Toner said that Obama laid out his vision for a nuclear-free world in a landmark address in Prague in 2009, and the administration has since achieved progress on a number of fronts, including reducing its own deployed stockpiles and launchers and diminishing the role of nuclear weapons in its security strategy.

"We are always looking for additional ways to achieve progress towards the president's goal, while maintaining, and this is important, a credible deterrent for the United States, our allies and our partners," Toner said at a regular briefing.

"We've said we'll continue to review our planned modernization programs, we're going to continue to assess whether there are additional steps that we can take to reduce the role of nuclear weapons in our security strategy and pursue ways to strengthen the global nonproliferation regime further. But, as I said, we're always going to maintain a credible deterrent for our friends and our allies," he said.

Japanese Prime Minister Shinzo Abe has reportedly expressed opposition to the "no first use" idea, raising concern that the policy could weaken deterrence against North Korea. Abe conveyed his concern to Adm. Harry Harris Jr., commander of the U.S. Pacific Command, according to reports.

South Korea's Foreign Ministry spokesman, Cho June-hyuck, said earlier Tuesday that the country believes the U.S. nuclear umbrella is "firm."

"The U.S. government has steadily been reaffirming through various occasions that the U.S. extended deterrence and security commitment to South Korea is firm and will remain unchanged going forward," he said at a press briefing. (Yonhap)