The United States has not ruled out the option of using nuclear weapons "first," due in part to North Korea's potential threat to South Korea, a former senior U.S. official said Tuesday, stressing that America's security commitment to its Asian ally remains credible.

During an interview with reporters at a forum, Robert Einhorn, former special adviser for nonproliferation and arms control at the U.S. State Department, also underscored the role of China in the successful implementation of international sanctions on the North. 

"One reason we have not ruled it out is because of the potential threat that North Koreans pose against South Korea," he said, pointing out that the U.S. has never adopted a "no-first-use" policy.

"So the U.S. has said that it is prepared if necessary to use nuclear weapons first -- whether in Europe or in East Asia -- to support South Korea and Japan. This remains U.S. policy," he added during the interview on the sidelines of the Asan Plenum 2016, an annual forum hosted by the Seoul-based Asan Institute for Policy Studies.

Einhorn made the remarks amid growing concerns here over America's security commitment to the South in the face of an increasingly provocative North Korea that has threatened the South and the U.S. through its evolving nuclear and missile programs.

The concerns escalated further after U.S. Republican presidential contender Donald Trump indicated the possibility of the withdrawal of American troops from the peninsula due to heavy costs for the upkeep of such forces. 

"The U.S. has 28,500 military personnel here in South Korea. We have the Combined Forces Command. We exercise jointly frequently. The military interactions between our two governments are intimate and they are stronger than ever," he said.

"So, I think the U.S. nuclear umbrella is credible today, and it would remain credible in the future -- as long as our alliance is strong and as long as the U.S. forces are deployed here."

Pointing out that diplomacy cannot succeed without pressure, Einhorn called for a dual-track approach that mixes diplomacy with pressure to persuade Pyongyang to renounce its nuclear ambitions.

In applying the pressure, China is the "key," he stressed. 

"Unless China implements conscientiously and vigorously, then sufficient pressure will not be placed on North Korea to give it incentives to negotiate seriously and to limit and eventually give up its nuclear capabilities," he said.

Commenting on President Park Geun-hye's planned trip to Iran, Einhorn, who also served as a U.S. negotiator on the Iranian nuclear issue, said that there are several messages the president needs to convey to the Islamic republic. Park is set to visit Tehran from May 1-3 with a focus on bolstering bilateral economic ties.

"Park should urge Iranians to show restraint in the ballistic missile activity area," he said, calling Iran's missile tests "destabilizing activities." 

He also said that Park should emphasize to the Iranian leadership that South Korea's security could be adversely affected by collaboration between North Korea and Iran on either nuclear or missile technologies.  (Yonhap)