North Korea fired unidentified projectiles twice Friday into the sea off its eastern coast in its third weapons tests in just over a week, South Korea's military said.

The North's increased testing activity is seen as brinkmanship aimed at increasing pressure on Seoul and Washington over the slow pace of nuclear negotiations.

South Korea's Joint Chiefs of Staff said the launches were conducted at 2:59 a.m. and 3:23 a.m. local time from North Korea's South Hamgyong Province into the East Sea.

"We are monitoring the situation in case of additional launches and maintaining a readiness posture," South Korea's Yonhap news agency quotes the Joint Chiefs of Staff as saying.

A U.S. official, speaking on condition of anonymity, said at least one projectile was detected that did not pose a threat to North America, although there could have been multiple projectiles.

The North fired two short-range ballistic missiles early Wednesday, only days after it launched two similar missiles on July 25.

U.S. President Trump played down the launches when asked about them at the White House just after news broke about the latest projectile launch. He told reporters he was not worried as they were short-range and "very standard."

Earlier on Thursday and before the latest launch, U.S. national security adviser John Bolton said North Korea's latest missile launches did not violate a pledge that North Korean leader Kim Jong-un made to Trump not to test long-range missiles and nuclear weapons.

The two leaders agreed at a June 30 meeting to revive stalled denuclearization talks, but efforts to resume the negotiations remain in doubt. Diplomats have criss-crossed the region this week in the hope of restarting the talks.

Experts say the North is demonstrating its frustration over planned U.S.-South Korea military exercises and stalled nuclear negotiations with the U.S., and its weapons tests could intensify if negotiations do not proceed rapidly over the next few months.

By firing weapons that directly threaten South Korea but not the U.S. mainland or its Pacific territories, North Korea also appears to be dialing up pressure on Seoul and testing how far Washington will tolerate its bellicosity without actually causing the nuclear negotiations to collapse, analysts say.

A summit between Trump and Kim in Vietnam in February collapsed after the two sides failed to reconcile differences between U.S. demands for North Korea's complete denuclearization and North Korean demands for sanctions relief.

On Thursday, U.S. Secretary of State Mike Pompeo said he hoped talks would start soon, though he "regretted" that a highly anticipated meeting with North Korean Foreign Minister Ri Yong-ho would not take place in Thailand this week.

This screenshot image taken from video broadcast by North Korea's KRT on Thursday shows a rocket test. (KRT via AP)

Ri has cancelled a trip to an Association of Southeast Asian Nations conference in Bangkok that Pompeo is attending.

"We stand ready to continue our diplomatic conversation," Pompeo told a news conference in Bangkok, adding that he was optimistic Kim would deploy his team for working-level talks "before too long."

At the United Nations on Thursday, Britain, France and Germany called on North Korea to engage in "meaningful" talks with the U.S. and said international sanctions needed to be fully enforced until Pyongyang dismantled its nuclear and ballistic missile programs.

Their statement came after a closed-door UN Security Council meeting on the latest launches.