They are believed to be ground-to-ship or ship-to-ship missiles with a range of some 100km (60 miles), they say. They landed in North Korean waters.
Pyongyang test-fired at least one short-range missile at the end of May - off its east coast - as part of what appeared to be military exercises.
North Korea's military programme is a major source of international concern.
"I believe North Korea launched two missiles into the West Sea today [Thursday] - one in the morning and the other in the afternoon," a South Korean intelligence official is quoted as saying by South Korean news agency Yonhap.
Thursday's suspected test off the west coast may have been carried out after a similar test failed on 25 May, when the North test-fired missiles off the east coast, a South Korean official was quoted as saying earlier.
"It's seen as the North firing the missile that it didn't launch at that time," he said.
The US condemned the launches, describing them as "not constructive".
"The United States and our allies believe that North Korea should refrain from testing missiles," said National Security Council spokesman Gordon Johndroe.
The North "should focus on the denuclearisation of the Korean peninsula and fulfil its obligations" under a February agreement to close down its only nuclear reactor, he added.
Missed deadline
South Korea confirmed at least one missile was test-fired into the Sea of Japan on 25 May, although earlier reports suggested several had been fired.
It coincided with the launch of South Korea's first destroyer equipped with US-supplied high-tech Aegis radar.
Despite international alarm at the North's long-range missile and nuclear tests last year, the US and Pyongyang's regional neighbours played down the May test.
The US state department described it as a "routine exercise" that would not affect six-party talks regarding the North's nuclear programme.
The tests come weeks after the North missed the February deadline to shut down its Yongbyon nuclear reactor in return for economic and energy aid.
Pyongyang has refused to move forward on the deal until it has access to North Korean funds in a Macau bank, which had been frozen amid claims the money was linked to counterfeiting and money-laundering.
North Korea test-fired several short-range missiles off its west coast, South Korea's Yonhap news service reported today.

"I believe North Korea launched two missiles into the West Sea today, one in the morning and the other in the afternoon," an unnamed South Korean intelligence official was quoted as saying by Yonhap, the South Korean government news service. The official added that the rockets were either a surface-to-ship type or a ship-to-ship type, both with a range of less than 100 kilometers (60 miles), and landed in North Korean territory.

"North Korea conducts this kind of missile test several times a year," the official was quoted as saying. "We saw the test as part of a routine military exercise."

The last missile test was on May 25, which at the time South Korean and US officials said was routine.

But the latest launches occurred a day after US President George W. Bush and Japanese Prime Minister Shinzo Abe called on the G8 nations to put more pressure on North Korea to comply with an international agreement to shut down its main nuclear reactor.

The U.S. criticized today's launch as "not constructive."

"The United States and our allies believe that North Korea should refrain from testing missiles," US National Security Council spokesman Gordon Johndroe said in a statement from the G8 summit in Heiligendamm, Germany. "North Korea should focus on the denuclearization of the Korean peninsula and fulfill its obligations," Johndroe said, mentioning an international agreement North Korea made on Feb. 13 that it would to shut down its main nuclear reactor in return for financial and diplomatic incentives.