The US says it has carried out a successful test of its controversial anti-missile system and has flown B-1 bombers over the Korean peninsula.
The exercises are a direct response to recent North Korean missile tests, which Pyongyang said proved that the entire US was within striking range.
But UN Ambassador Nikki Haley said there was no point in an emergency UN Security Council meeting because it would produce “nothing of consequence”.
She urged China to rein in North Korea.
“An additional Security Council resolution that does not significantly increase the international pressure on North Korea is of no value,” Ms Haley said in a statement.
“In fact it is worth than nothing because it sends the message to the North Korean dictator that the international community is unwilling to seriously challenge him.
“China must decide whether it is finally willing to take this vital step. The time for talk is over,” she added.
Despite fierce objections from China, the US military has begun installing the Thaad system in South Korea with the aim of shooting down any North Korean missiles fired at the South.
US B-1 bombers also conducted exercises over the Korean peninsula with South Korean and Japanese planes.
On Friday, North Korea test-fired a second intercontinental ballistic missile (ICBM).
The launch came three weeks after the state’s first ICBM test.
On Saturday, US President Donald Trump criticised China for not doing enough to stop Pyongyang’s weapons programme while making “billions of dollars” in trade with North Korea.
Mr Trump wrote on Twitter that he was “very disappointed” with China, adding that he would not allow the country to “do nothing” about the isolated state.
But Victor Gao, a former diplomat and Chinese government adviser, said that Mr Trump’s comments were unhelpful, adding that the US was acting like a “spoiled child”.
China, which shares a land border with North Korea and is its closest economic ally, earlier condemned the North’s test launch and urged restraint on all sides.
Mr Trump and his Chinese counterpart Xi Jinping discussed North Korea during talks earlier this year, after which US officials said the two countries were working on “a range of options”.
But Friday’s ICBM launch demonstrates a defiance from the North, which is showcasing “a significant advancement in technology”, South Korea said.