(CNN) — South Korea scrambled about 80 fighter jets after detecting a large number of North Korean warplanes during a four-hour period Friday, the country’s military said, in a further escalation of regional tensions.

In a statement, the South Korean military said it spotted about 180 North Korean military aircraft between 11 a.m. and 3 p.m. local time, a day after Pyongyang is believed to have conducted the failed test of an intercontinental ballistic missile (ICBM).

Tensions in the Korean Peninsula began rising Monday, when the “Vigilant Storm” joint military drills began between the United States and South Korea, involving hundreds of aircraft and thousands of service members from both countries, according to the US.

North Korea accused the allies of provocative action and on Wednesday launched 23 missiles from its east and west coasts — the most missiles it’s fired in a single day — into waters either side of the peninsula, prompting Seoul to respond with three surface-to-air missiles.

Friday’s South Korean deployment included an unspecified number of F-35A stealth fighter jets, the statement said, and the South Korean warplanes participating in the ongoing joint maneuvers had also “maintained a readiness posture,” the South Korean military said.

After Thursday’s suspected ICBM test, the US and South Korea announced they’d extend the drills for an extra day until November 5, a move denounced by a North Korean official as a “very dangerous and false choice,” according to state media.

Later, after meeting with his South Korean counterpart at the Pentagon, US Defense Secretary Lloyd Austin accused North Korea of “irresponsible and reckless activities.”

“We’ve said before these kinds of activities are destabilizing to the region potentially. So we call on them to cease that type of activity and to begin to engage in serious dialogue,” Austin said.

At a United Nations Security Council meeting on Friday, which numerous countries had called to discuss Pyongyang’s missile launches, the US Ambassador to the UN accused the council of inaction.

“For a UN Member State to so flagrantly violate the Security Council resolutions, and all that the UN Charter stands for, is appalling,” said Linda Thomas-Greenfield. “Equally appalling is the Council’s deafening silence on this issue.”

She delivered a joint statement on behalf of 12 member nations, including South Korea, the US and the United Kingdom, condemning the launches as a regional and international threat.

In an interview on CNN on Wednesday, Thomas-Greenfield said the UN would be “putting pressure” on China and Russia to improve and enhance sanctions against North Korea. She declined to say whether US President Joe Biden would raise sanctions with China’s President Xi Jinping at the G20 but said it was “on the President’s mind.”