BEIJING >> North Korea’s main ally China said it “firmly opposes” Pyongyang’s purported hydrogen bomb test and is monitoring the environment along its border with the North near the test site.

China plans to summon North Korea’s ambassador in Beijing to the Foreign Ministry to lodge a strong protest, spokeswoman Hua Chunying told reporters at a daily briefing Wednesday. China made a similar protest after the North’s last nuclear test in 2013.

Wednesday’s reported test was performed in defiance of the international community and in contravention of its earlier denuclearization promises, Hua said.

“North Korea should stop taking any actions which would worsen the situation on the Korean peninsula,” Hua said.

Environment bureau technicians were monitoring conditions near the border but air quality near the bomb site was within the normal range, Hua said.

In the longer term, North Korea should return to long-stalled six-nation denuclearization talks hosted by China, Hua said. North Korea abandoned that process in 2009, saying it would continue its nuclear program to produce a deterrent against alleged threats from the U.S. and other enemies.

While she made no mention of measures to respond to a test, analysts say Pyongyang’s proceeding against Beijing’s objections would seriously harm a relationship already under considerable strain.

That will likely include agreeing to tougher U.N. sanctions against Kim Jong Un’s hard-line communist regime and possibly unilateral trade restrictions that could hurt the North’s moribund economy.

Despite that, China remains the North’s most important ally and trade partner.