North Korea said Friday it test-fired a new anti-aircraft missile, its fourth weapons launch in recent weeks, which experts say is part of a strategy to receive relief from economic sanctions and win other concessions.

South Korea, Japan, and the United States typically publicly confirm North Korean ballistic missile launches, which are banned by U.N. resolutions, soon after they occur. But they did not do so for Thursday’s launch, indicating the weapon tested may have been a different kind. Seoul’s Joint Chiefs of Staff said Friday that South Korean and U.S. intelligence authorities monitored moves by North Korea but didn’t elaborate.

Three weeks ago, North Korea resumed missile tests after a six-month lull. As it has sometimes done before, North Korea combine sow of force with a more conciliatory gesture, offering earlier this week to reactivate hotlines that North and South Korea use to set up meetings, arrange border crossings and avoid accidental clashes.

Diplomacy aimed at getting North Korea to abandon its nuclear arsenal in return for economic and political rewards has largely been deadlocked since early 2019. That has left North Korea under crippling U.S.-led economic sanctions at a time when its fragile economy is suffering massive setbacks due to the coronavirus pandemic.

North Korea’s latest moves appear aimed at pressuring South Korea, which wants to improve strained ties on the peninsula, to persuade the U.S. to relax the sanctions.

On Friday, the North’s Korean Central News Agency said the anti-aircraft missile test was “of very practical significance in studying and developing various prospective anti-aircraft missile systems.”

Kim Dong-yub, a professor at the University of North Korean Studies in Seoul, said the launch appears to be the primitive stage of a test to develop a missile designed to shoot down incoming enemy missiles and aircraft. He said the missile resembles the Russian-made S-400 air defense system, which he said has a maximum range of 400 kilometers (250 miles) and is reportedly capable of intercepting stealth jets.

The U.N. Security Council received a briefing on the recent launches and the humanitarian and COVID-19 situations in North Korea at an emergency closed-door meeting Friday but took no action.

(With inputs from AP)