South Korea's foreign ministry on Thursday raised the terrorism alert level for five diplomatic offices in the region citing intelligence that North Korea may attempt to harm its officials.

The five locations include Seoul's embassies in Cambodia, Laos and Vietnam, as well as consulates in Vladivostok, Russia, and Shenyang, China, the ministry said in a statement.

The terrorism alert level was raised from Attention to Alert, the second highest among South Korea's four classifications, which indicates the chances of an attack are strong, the foreign ministry said.

Separately, South Korea's National Intelligence Service (NIS) said it had a "number of indications that North Korea is preparing to carry out terrorist attacks against our diplomatic officers and citizens," but did not elaborate on the nature of the threats.

Pyongyang has dispatched agents to those countries to tighten surveillance of the South Korean missions, the NIS said.

The North Korean embassy in London did not respond to repeated phone calls for requests for comment.

The North's government-controlled media has criticized allegations of terrorism against it as U.S.-led efforts to discredit opponents of Washington.

The foreign ministry statement also said South Korea's National Counter Terrorism Center held a meeting on Thursday to discuss measures to protect the diplomatic offices and officials who work there.

During the Cold War, North Korea was accused of carrying out several attacks on civilian targets, including bombings at a Seoul airport and a South Korean airliner in the 1980s.

The United States placed North Korea back on the list of state sponsors of terrorism in 2017, citing the killing of Kim Jong Nam, the older half-brother of North Korean leader Kim Jong Un, which was carried out with VX nerve agent at an airport in Malaysia. (Reporting by Hyunsu Yim; Editing by Josh Smith and Christian Schmollinger)