(CNN) -- North Korea called a joint military drill among U.S., South Korea and Japan "reckless military provocations" that could lead to "nuclear war" as the allies started their operations Thursday amid tensions in the region.
USS George Washington roared into the sea off the coast of South Korea to conduct a drill with the two allies.
A number of destroyers and other naval vessels will take part in the two-day exercise that will focus on reconnaissance, search and seizure, and rescue missions.
The trilateral exercise will be followed by a separate U.S./South Korea drill and routine carrier operations between the two navies.
North Korea's official state media KCNA on Wednesday urged the U.S. and South Korea to "think twice over the grave consequences" of the drills and South Korea's arms upgrades.
"Even the slightest provocation on the peninsula may promptly lead to an all-out nuclear war between both sides and a regional war because it is the region where the world's biggest military forces including nukes are concentrated and there is neither lasting mechanism nor guarantee for averting a war," KCNA said.
"The U.S. and the Lee group would be well advised to think twice over the grave consequences to be entailed by their moves for a new war," it added, referring to South Korean President Lee Myung-bak.
The trilateral naval drill is not their first. The navies have previously conducted trilateral activities since 2008, according to public affairs office of Naval Forces in Korea.
Nevertheless, the joint exercise will boost cooperation between the three allies, according to Park Chang Kwoun, senior research fellow at Korea Institute for Defense Analyses.
"Because of historical issues, military cooperation between South Korea and Japan were not conducted very smoothly. But the drill will perform as a framework in further improving the military relationship," Park said.
The series of drills will pressure North Korea, said the former commander of Korean Defense Intelligence Command, a military agency that closely monitors North Korea.
"This year, we have seen frequent harsh words from the North. We can expect them to respond with condemnation, but given nature of the drill and how they had reacted to the joint drills in the past, any provocations are unlikely," he said.
The two Koreas are still technically at war after the 1950-53 conflict.
Tension in the Korean peninsula has escalated since North Korea's shelling of a South Korean island in 2010, which killed four, and sinking of a South Korea warship that killed 46 sailors in a torpedo attack blamed on the North. The North has denied it was behind the attack.
CNN's Yoko Wakatsuki contributed to this report