South Korea said DPRK fired on Friday an "unidentified ballistic missile," hours after the White House claimed that Pyongyang had delivered arms to Russia.

"North Korea fires an unidentified ballistic missile into (the) East Sea," South Korea's Joint Chiefs of Staff confirmed, referring to the body of water also known as the Sea of Japan.

The launch comes after a year of unprecedented weapons tests by North Korea, including the launch of its most advanced intercontinental ballistic missile the month before.

On Sunday, Seoul's military also said that the DPRK fired two ballistic missiles, days after Pyongyang tested a "high-thrust solid-fuel motor," with state media describing it as an important test "for the development of another new-type strategic weapon system."

The United States and South Korea have claimed for months that Pyongyang is preparing to conduct its seventh nuclear test.

On Tuesday, Washington and Seoul held a joint air drill and deployed a US B-52H strategic bomber to the Korean peninsula, the South Korean Joint Chiefs of Staff said in a statement.

The long-range heavy bomber was part of an exercise that included the US and South Korea's most advanced jets -- including the F-22 and the F-35 stealth fighters.

Despite heavy international sanctions over its weapons programs, Pyongyang has built up an arsenal of intercontinental ballistic missiles (ICBMs).

North Korean leader Kim Jong Un has placed strategic priority on developing solid-fuel engines for more advanced missiles.

Kim said this year that he wants the DPRK to have the world's most powerful nuclear force and declared his country an "irreversible" nuclear state.

The wishlist he revealed last year included solid-fuel ICBMs that could be launched from land or submarines.

The DPRK has conducted dozens of missile tests this year, including multiple launches of ICBMs with potential range to the US mainland and an intermediate-range missile flown over Japan.

The launches came in response to the expansion of the allies' combined military exercises, which DPRK insists are rehearsals for an invasion.

Read more: DPRK rebuffs claims of alleged weapons supplies to Russia: Yonhap