North Korea on Wednesday fired several unidentified projectiles off its east coast, South Korea's military said, less than a week after the North launched two short-range ballistic missiles into the sea.

Observers say the launches were aimed at ramping up pressure on the United States to make concessions as the two countries are struggling to resume diplomacy on the North's nuclear weapons program.

South Korea's Joint Chiefs of Staff said in a statement the latest launches were done from the North's Hodo peninsula on the east coast, a regular weapons launch site. It said South Korea's military is monitoring for possible additional launches by North Korea.

It wasn't immediately known exactly what North Korea fired or how far the projectiles flew.

The launches came six days after North Korea fired two short-range ballistic missiles that Seoul officials say flew 600 kilometres before landing off the North's east coast.

The latest launches also came hours after a senior U.S. official said President Donald Trump has sent mementos from his brief visit to North Korea last month to Kim Jong-un.

The official said a top staffer from the National Security Council hand-delivered photographs from the June Trump-Kim meeting at the demilitarized zone to a North Korean official last week. The Trump administration official spoke on the condition of anonymity because the official wasn't authorized to speak publicly.

The DMZ meeting was the third summit between Trump and Kim since they first met in Singapore last year. Diplomacy between North Korea and the U.S. remained deadlocked since the second Trump-Kim summit in Vietnam in February ended without any agreement. In Vietnam, Trump rejected Kim's demand for widespread sanctions relief in return for dismantling his main nuclear complex, a partial disarmament step.

During the DMZ meeting, Trump and Kim agreed to resume the nuclear diplomacy in coming weeks, but there hasn't been any known meeting between the countries. Some experts say North Korea wants a U.S. promise to ease sanctions, accept a slow, step-by-step disarmament process by North Korea or make other concessions once the diplomacy restarts. Despite a recent lack of progress in nuclear diplomacy, both Trump and Kim have said they have maintained good relations with each other.

'Solemn warning'

North Korea's state media said Kim supervised a test of a new missile last Thursday designed to deliver "solemn warning" to South Korea over its purchase of high-tech U.S.-made fighter jets and its plans to conduct military drills that Pyongyang sees as an invasion rehearsal.

South Korea's military said the flight data of the weapon launched Thursday showed similarities to the Russian-made Iskander, a short-range, nuclear-capable missile. A North Korean version could likely reach all of South Korea — and the 28,500 U.S. forces stationed there — and would be extremely hard to intercept.

The North Korean government released an image showing a test launch last Thursday. (Korean Central News Agency/Korea News Service via AP)

After Thursday's missile launches, Trump tried to downplay the significance of the tests, saying that "short-range" was the most important word. He said North Korea fired "standard" missiles many countries possess.

Thursday's launches were a violation of UN Security Council resolutions that ban North Korea from engaging in any launch using ballistic technology. While the North could face international condemnation over the latest launches, however, it's unlikely that the nation, already under 11 rounds of UN sanctions, will be hit with fresh punitive measures. The UN council has typically imposed new sanctions only when the North conducted long-range ballistic launches.