The United States has called on China to end "business as usual" with its ally North Korea after Pyongyang defied world powers by saying it had tested a hydrogen bomb.
And South Korea says it will retaliate by broadcasting propaganda across the border.
South Korea, which has grown increasingly close to China in recent years, says its foreign minister will speak with his Chinese counterpart later on Friday.
US Secretary of State John Kerry said on Thursday he had made clear in a phone call with Chinese Foreign Minister Wang Yi that China's approach to North Korea has not succeeded.
"China had a particular approach that it wanted to make, that we agreed and respected to give them space to implement that," Kerry told reporters.
"Today in my conversation with the Chinese I made it very clear that has not worked and we cannot continue business as usual."
China is the North's main economic and diplomatic backer, although relations between the two Cold War allies have cooled in recent years.
China's foreign ministry said after the call with Kerry that Beijing was willing to communicate with all parties, including the United States.
"Wang Yi stressed that China has staunchly dedicated itself to the goal of the peninsula's denuclearisation and to maintaining peace and stability on the peninsula," the Chinese Foreign Ministry said in a short statement.
South Korea's foreign ministry had requested a phone call with Wang since directly after North Korea announced on Wednesday it had tested a hydrogen bomb, the South's Yonhap News Agency said.
However, it said the call had been delayed due to China's "internal scheduling," citing an unnamed official.
South Korea's propaganda broadcasts by loudspeaker across the heavily militarised border, known to infuriate the leadership of isolated North Korea, were due to begin early on Friday.
The last time Seoul deployed the speakers, in retaliation for a landmine blast in August that wounded two South Korean soldiers, it led to an armed standoff and exchange of artillery fire.
South Korea heightened military readiness to its highest level at locations near the loudspeakers, and Seoul vowed to retaliate against any attack on the speakers.
The South Korean city of Paju, which sits along the border with North Korea, suspended tours of the Demilitarised Zone (DMZ) at the military's request.
Seoul has also raised South Korea's cyber security alert level while Yonhap also reported that North Korea had boosted troop deployments and raised its surveillance of the South.
Wednesday's test angered both the United States and China, which was not given prior notice, although the US government and weapons experts doubt Pyongyang's assertion that the device it exploded was a hydrogen bomb.
Kerry said he and Wang agreed to work closely to determine what measures could be taken given increasing concerns about the nuclear test.
Meanwhile, US Republicans and Democrats in the House of Representatives could join forces in a rare display of unity to further tighten sanctions on North Korea.
Nancy Pelosi, the House Democratic leader, told reporters that Democrats would support a North Korea bill likely to be brought for a vote by Republicans next week.
A congressional source said it was expected as soon as Monday.
The House measure would target banks facilitating North Korea's nuclear program and authorise freezing of US assets of those directly linked to illicit North Korean activities. It would also penalise those involved in business providing North Korea with hard currency.
It was unclear, however, how more sanctions would deter North Korea, which has conducted four nuclear tests since 2006 while paying little heed to international pressure.
Reuters
