South Korean residents take shelter at the South Korea-controlled island of Yeonpyeong on Monday after North Korea started a live-fire drill. Photo: AFP/Yonhap

North Korea and South Korea on Monday exchanged fire in the sea across the western maritime border, South Korea-based Yonhap news agency reported.

North Korea has fired several artillery shells in waters north of the Northern Limit Line (NLL), or the disputed sea border, amid its live-fire drill started in the morning. Some shells landed south of the NLL in South Korea's territorial waters and Seoul responded with its own artillery fire, according to the Joint Chiefs of Staff (JCS).

South Korean military said the number of the shells it fired should be equal to that from North Korea that landed into South Korea 's territorial waters, Yonhap quoted JCS as saying, adding that Seoul's army, navy and air force has declared a state of emergency.

South Korea has also evacuated border island residents amid the live-fire drill.

North Korea declared a no-navigation zone in waters near the western maritime border earlier in the day, banning ships from sailing near the NLL.

China on Monday expressed concern about the rise of tensions on the Korean Peninsula, following South Korea and North Korea dispute on the western maritime border.

"We urge all relevant parties to maintain calm, exercise restraint and refrain from acts going against peace and stability on the peninsula," said foreign ministry spokesman Hong Lei at a daily press briefing.

The current situation on the peninsula is quite vulnerable, and safeguarding peace and stability conforms with the common interests of all relevant parties, he said.

Hong reiterated China's position to realize denuclearization on the Korean Peninsula, safeguard peace and stability there, and solve the issues through talks and consultations.

The UN Security Council issued a statement on Friday, condemning the North Korean recent ballistic missile launches.

As a response, the North Korean foreign ministry Sunday said that it will "not rule out a new form of nuclear test" to bolster its nuclear deterrence.