President Barack Obama says the United States is trying to "keep a door open" for North Korea to return to six-party nuclear disarmament talks.
The president's comments came in an interview with the Associated Press on Thursday, the same day North Korea test-fired four short-range missiles.
A spokesman for South Korea's defense ministry says two ground-to-ship missiles were fired within a 40-minute span Thursday near the eastern coastal city of Wonsan. The ministry said the missiles landed about 100 kilometers away in the Sea of Japan.
South Korean officials said two more missiles were fired a few hours later from the east coast.
The United States has urged North Korea to stop its provocative actions and return to the nuclear disarmament talks.
In recent months, Pyongyang also has tested a nuclear weapon and a ballistic missile.
President Barack Obama told the Associated Press there could be more sanctions in store for North Korea for its May nuclear test.
State Department spokesman Ian Kelly said the international community "has spoken loudly" and that North Korea knows it has to halt the testing.
Japanese Prime Minister Taro Aso responded to North Korea's missile tests Thursdayby saying his government has repeatedly warned Pyongyang that such acts are not in North Korea's national interest.
The development comes as tensions increase on the Korean peninsula over North Korea's May 25 nuclear test and a series of missile launches. The United Nations Security Council imposed a harsh new set of financial sanctions against the regime last month to prevent it from obtaining material and equipment needed for nuclear and arms programs.
U.S. envoy Philip Goldberg met with Chinese officials in Beijing Thursday to discuss rigorous implementation of sanctions. The U.S. State Department says Goldberg also will meet with Malaysian officials in Kuala Lumpur before returning to Washington July 6.
Chinese Vice Foreign Minister Wu Dawei is leaving Beijing Thursday to hold talks with nuclear negotiators in Russia, Japan, South Korea and the United States in an effort to restart the six-nation talks.
The last round of talks in December in Beijing ended with an impasse over how North Korea's disarmament would be verified.
Some information for this report was provided by AFP, AP and Reuters.