– The U.S. Navy is moving a sea-based radar platform closer to North Korea to track possible missile launches, a Pentagon official said Monday, in the latest step meant to deter the North and reassure South Korea and Japan that the U.S. is committed to their defense.

The sea-based X-band radar, a self-propelled system resembling an oil rig, is heading toward the Korean peninsula from Pearl Harbor, the official said. The John S. McCain, a guided missile destroyer capable of shooting down ballistic missiles, also is being sent to the region, said another Defense Department official. The officials spoke on condition of anonymity in order to discuss ship movements.

The Pentagon’s decision to send only two fighters appeared to reflect a delicate balance, seeking to demonstrate U.S. resolve while also seeking to ease tensions after a fraught few days in which the North Korean leader threatened to rain missiles on the U.S. mainland and the United States responded by flying nuclear-capable bombers over the Korean Peninsula and pledging to spend $1 billion to expand ballistic missile-defense systems along the Pacific Coast.

Though U.S. officials said they remained concerned about the invective flowing from North Korea — and South Korea’s president ordered military commanders to carry out a swift and strong response to any provocations “without any political consideration” — the Obama administration said there was no evidence that the country’s leader, Kim Jong Un, was mobilizing troops or other military forces for any imminent attack. “We are not seeing changes to the North Korean military posture such as large-scale mobilizations or positioning of forces,” said Jay Carney, the White House press secretary. “What that disconnect between rhetoric and action means, I’ll leave to the analysts to judge.”

Having taken unusually public steps to demonstrate its commitment to defend itself and protect South Korea and Japan, the Obama administration appeared to be trying to defuse a situation that many analysts say has gone beyond previous cycles of provocation by North Korea, and raised genuine fears of war.

“It is a calculated response to say, ‘We don’t want anyone to think the situation is getting out of control, that the ladder of escalation is going to end in a full-scale conflict,’ ” said Jeffrey Bader, who worked on North Korea policy for the Obama administration from 2009 to 2011.

Still, South Korean President Park Geun-hye ordered her country’s military to deliver a strong and immediate response to any North Korean provocation. “I consider the current North Korean threats very serious,” Park told the South’s generals. “If the North attempts any provocation against our people and country, you must respond strongly at the first contact.”

Park’s comments contrasted with the usually dismissive tone that South Korean leaders take toward the North’s threats, and reflected the criticism directed at her predecessor, Lee Myung-bak, when the South was seen as not retaliating after North Korea aimed an artillery barrage at a South Korean island in 2010, killing four people. Park’s campaign last year focused on a promise not to be blackmailed by the North.

Since Kim took power in late 2011, the North has launched a three-stage rocket, tested a nuclear device and threatened to hit major U.S. cities with nuclear-armed ballistic missiles. The peninsula, Kim declared, has reverted to a “state of war.”

His actions, analysts say, reflect the North’s growing frustration that its strategy of using threats and provocations against the United States and South Korea seemed less effective in recent years. Instead, the allies spearheaded another round of U.N. sanctions.

The U.S. vessel deployed to waters off the Korean Peninsula, an Aegis cruiser, will remain there “for the foreseeable future,” said a Defense Department official.

While analysts generally praise the administration’s handling of the tensions, Joel Wit, a former State Department official, said, “It’s starting to feel like we send a new airplane to South Korea every day to prove our resolve.” He said that Washington needed to explore diplomatic channels as well.

But Michael J. Green, a policy adviser in President George W. Bush’s administration, said that given the lack of success in curbing North Korea’s nuclear and missile programs, “the focus for the near future has to be on deterring North Korea, punishing them for violations, and constraining their ability to move their nuclear program forward or proliferate.”

The New York Times contributed to this report.