Seoul, South Korea (CNN) -- South Korea and the United States started joint military exercises Sunday, U.S. Forces Korea spokesman David Oten told CNN.
The drills are taking place amid heightened tensions between the Koreas after North Korea warned of unpredictable "consequences" if the United States fulfilled its vow of deploying an aircraft carrier to the Yellow Sea for the military maneuvers.
Four South Koreans died after North Korea's shelling of Yeonpyeong Island during a South Korean military drill Tuesday. North Korea said the South provoked the attack because shells from the South Korean drill landed in the North's waters.
Earlier Sunday, the United States and South Korea began assembling ships for exercises off the west coast of the Korean peninsula in the Yellow Sea, a source at the South Korean Joint Chiefs told CNN.
Also on Sunday, South Korean defense officials said the country had issued and lifted an order for civilians to take cover in shelters on the border island of Yeonpyeong after hearing loud noises inside North Korea that sounded like rounds of artillery firing, though no shells landed on the island.
North Korea's official Korean Central News Agency warned Sunday what could happen if the country perceives its waters are infringed upon.
"The DPRK [Democratic People's Republic of Korea] will deal a merciless military counter-attack at any provocative act of intruding into its territorial waters in the future," the state news agency said.
China's foreign minister spoke with his Russian, U.S., and Japanese counterparts, and a Chinese representative visited Seoul as envoys underscored the need to lower the temperature in the longtime flash-point region.
The KCNA has slammed South Korea and the United States for provoking the crisis.
It called reports of civilian casualties part of South Korea's "propaganda campaign" and accused the "enemy" of creating "a human shield by deploying civilians around artillery positions and inside military facilities before the launch of the provocation."
"If the U.S. brings its carrier to the West Sea of Korea at last, no one can predict the ensuing consequences," said KCNA, referring to the aircraft carrier USS George Washington, which is set to join South Korea's forces near the coasts of China and North Korea for the four-day military drill scheduled to start Sunday.
U.S. State Department spokeswoman Nicole Thompson called the claims "outrageous."
"This is just another example of North Korea's own internal propaganda. The North Koreans for many years, including the Cheonan warship incident, have taken provocative action. This didn't have anything to do with U.S. actions," Thompson told CNN, referring to the sinking of a South Korean ship in March that left 46 people on board dead.
The United States and South Korea blame the sinking on the North, which has consistently denied responsibility.
Diplomats, seeking a lessening of tensions and a return to the six-party talks with North Korea over the country's nuclear aspirations, have busily labored to avert more hostilities. The United States, China, Japan, Russia, South Korea and North Korea are the six countries that have been involved in the talks, which were put on hold in 2008.
"These parties should call on the DPRK and South Korea to exercise calmness and restraint and hold dialogue and make contacts, and not to take actions that would escalate the conflict," China's official Xinhua news agency quoted Chinese Foreign Minister Yang Jiechi as saying. China is North Korea's largest trading partner.
Yang and Russian Foreign Minister Sergey Lavrov "stressed the need to prevent the situation from exacerbating and to work toward relieving the tensions," according to the Russian Foreign Ministry.
Xinhua reported that Japanese Foreign Minister Seiji Maehara said his country "is willing to work together with China to joint safeguard peace and stability on the Korean peninsula."
And a Twitter message from U.S. State Department spokesman P.J. Crowley said Secretary of State Hillary Clinton spoke with Yang on Friday and "encouraged Beijing to make clear that North Korea's behavior is unacceptable."
Meanwhile, Dai Bingguo, a Chinese state councilor, sat down with South Korean Foreign Minister Kim Sung-hwan in Seoul to discuss the tensions.
The violence has sparked anger and political turmoil in South Korea. The country's defense minister, Kim Tae-young resigned after the exchange of fire, and veterans of the South Korean military protested Saturday on the streets of Seoul, stating they were angry that their country's government had not done enough to respond to the North's shelling.
One group of protesters gathered near the defense ministry building Saturday, clashing with police officers with some charging and kicking officers.
Two South Korean marines were among the four killed in the shelling. Hundreds of mourners attended their nationally televised funeral Saturday, weeping before photos of the two men set among an array of flowers.
As for Sunday's joint military exercises, China appeared to criticize them Friday and Chinese analysts warned against the United States and South Korea embarking on "sensitive and provocative military actions."
"We oppose any party to take any military acts in our exclusive economic zone without permission," Foreign Ministry spokesman Hong Lei said in a statement, Xinhua reported.
But the United States has described the drill as defensive in nature. The exercises were planned months ago, and are meant to underscore strong ties between South Korea and the United States, defense officials from both countries have said.
There will be no live firing element in the drills. Live firing exercises can only take place in a designated training range or in a closed-off area at sea, said Cmdr. Jeff Davis, public affairs officer for the U.S. 7th Fleet. Such firing exercises are not possible given the amount of traffic in the area, he said.
The drills will include anti-air-attack and anti-surface-attack exercises, communications and data drills, expert exchanges, logistical support, and replenishment drills. For example, a Korean oil tanker will refuel a U.S. ship, Davis said.
But the prospect of more violence has prompted alarm across the region. Japan's Kyodo news agency reported that Japanese "Cabinet members have been ordered to stay in Tokyo until Wednesday and be at their ministry offices within an hour in the event emergency situations develop."
The tense maritime border between the two Koreas has become the major military flash point on the Korean peninsula in recent years.
The Yeonpyeong attack was the first direct artillery assault on South Korea since 1953, when an armistice ended fighting, though both Koreas are still technically at war.
