SEOUL: A nuclear-powered United States aircraft carrier and its battle group began exercises with South Korean warships on Monday, hours after North Korea fired two short-range ballistic missiles in an apparent protest of the allies’ expanding drills.

The seventh missile test this month underscored heightening tensions in the region as both the North’s weapons tests and the US-South Korea joint military exercises have intensified in a cycle of tit-for-tat.

The launches may have been timed for the arrival of the USS Nimitz and its strike group, including a guided missile cruiser and two destroyers, which engaged in air defense exercises and other drills with South Korean warships in waters near Jeju island.

Jang Do Young, a South Korean navy spokesman, said the drills were aimed at sharpening joint operational capabilities and demonstrating the US’ commitment to defend its ally with the full range of options, including nuclear, in face of the North’s “escalating nuclear and missile threats.”

The Nimitz strike group was expected to arrive in the South Korean mainland port of Busan on Tuesday.

South Korea’s Joint Chiefs of Staff said the two North Korean missiles were fired from a western inland area south of the North’s capital Pyongyang from about 7:47 to 8 a.m. (6:47 to 7 a.m. in Manila) and traveled around 370 kilometers (229 miles) before landing at sea.

Tokyo’s military said the missiles, which landed outside Japan’s exclusive economic zone, flew on an irregular trajectory and reached a maximum altitude of 50 km (31 mi).

Japan has previously used similar language to describe a North Korean solid-fuel missile apparently modeled after Russia’s Iskander mobile ballistic system, which is designed to be maneuverable in low-altitude flight to better evade South Korean missile defenses. North Korea also has another short-range system with similar characteristics that resemble the US MGM-140 Army Tactical Missile System.

Japanese Chief Cabinet Secretary Hirokazu Matsuno said North Korea might dial up its testing activity further with more missile launches or even conduct its first nuclear test since September 2017.

The South Korean and Japanese armed forces denounced the latest launches as a serious provocation threatening regional peace and said they were working with the US to analyze the missiles further. The US Indo-Pacific Command said the launches did not pose an immediate threat to Washington or its allies, but still highlighted the “destabilizing impact” of Pyongyang’s illicit nuclear weapons and missiles programs.

North Korea, which has weathered tightened United Nations Security Council sanctions since 2016 over its nuclear developments, didn’t immediately comment on the launches.

The US and South Korea completed their biggest springtime exercises in years last week, which included both computer simulations and live-fire field exercises. But the allies have continued their field training in a show of force against the North’s rising threats.

North Korea had also fired a short-range missile when the USS Ronald Reagan and its battle group arrived for joint drills with the South last September, which was the last time the US sent an aircraft carrier to waters near the Korean Peninsula.

North Korea has launched more than 20 ballistic and cruise missiles on 11 occasions this year as it tries to force the US to accept its nuclear status and negotiate the removal of sanctions from a position of strength.

The weapons tested this month included an intercontinental ballistic missile and a series of short-range missiles intended to overwhelm Seoul’s defenses as North Korea tries to demonstrate an ability to conduct nuclear strikes on both the South and the US mainland.

The North last week carried out what it described as a three-day exercise that simulated nuclear attacks on South Korean targets.

North Korean leader Kim Jong Un has condemned the US-South Korean joint military drills as invasion rehearsals. The allies say the exercises are defensive in nature.

The tests also included a purported nuclear-capable underwater drone that Pyongyang claimed can set off a huge “radioactive tsunami” and destroy naval vessels and ports. Analysts were skeptical whether such a device was a major new threat, and Seoul’s Joint Chiefs of Staff said in a statement on Monday that it considered the North Korean claims likely to be “exaggerated and fabricated.”