A US army OH-58D Kiowa Warrior helicopter makes a maneuver in a US-ROK joint live-fire military exercise at a training field in Pocheon, south of the demilitarized zone separating the Korean Peninsula, on Friday. Kim Hong-Ji / Reuters

The Republic of Korea and the United States kicked off their largest joint air drill on Friday at a time of elevated military tensions with the Democratic People's Republic of Korea.

The so-called "Max Thunder" exercise, which is held twice a year, will last until April 25 and involve 103 aircraft and 1,400 service personnel, an ROK air force spokesman said.

"In numerical terms, it's the largest exercise of its kind that we've done," the spokesman said.

The last Max Thunder drill held in October-November last year involved 97 aircraft and some 1,000 troops.

Seoul's F-15K jet fighters will take part along with US air force F-15 and F-16 and US Marines' FA-18 and EA-18 aircraft.

"The combined air forces will strengthen their battle readiness under the current situation when tension rises over the Korean Peninsula," an ROK air force statement said.

The exercise will focus on "practical scenarios" involving precision attacks on enemy positions and supply-drop missions for troops infiltrating enemy territory.

It comes as the allies are winding up separate annual military drills that began in late February, and have been denounced by the DPRK as rehearsals for invasion.

In a pointed protest at the exercises, the DPRK carried out a series of rocket and missile launches, capped by its first midrange missile test since 2009 on March 26.

The two neighbors also traded artillery fire across the tense Yellow Sea border on March 31, after Pyongyang dropped around 100 shells across the maritime boundary during a live-fire drill.

The exchange followed a DPRK warning that it might carry out a "new" form of nuclear test - a possible reference to a uranium-based device or a miniaturized warhead small enough to fit on a ballistic missile.

The US, the ROK and Japan, meeting in Washington on Monday, condemned the ballistic missile tests and urged the DPRK "to refrain from further threatening actions".

However, the DPRK accused the US of hypocrisy on Friday for remaining silent over a ROK missile test while condemning Pyongyang's recent launches.

"The US double-dealing attitude and despicable mode of action has been brought to light," a spokesman for the DPRK National Defense Commission said in a statement.

The statement, carried by the KCNA news agency, comes at a time of elevated military tensions.

"As long as the US persists in its hostile policy ... according to its high-handed, arbitrary and gangster-like double standards, we will push ahead with countermeasures for self-defense to put an end to the policy," the spokesman said.

His remarks were triggered by the ROK's test-firing last month of a new ballistic missile capable of carrying a one-ton payload to any part of the DPRK.