The top US commander in the Pacific has said an advanced missile defence system in South Korea aims to bring the North Korean leader Kim Jong-un "to his senses, not to his knees".
Adm Harry Harris said the US would be ready "with the best technology" to defeat any missile threat.
The White House has taken the rare step of calling senators to a classified briefing on North Korea.
Democrat Chris Coons said afterwards that it had been "sobering".
While unusual, the step to hold the meeting does not necessarily mean a crisis is imminent.
The US has deployed warships and a submarine to the Korean peninsula as well as its Terminal High-Altitude Area Defense (Thaad) missile defence system.
China argues the deployment of Thaad will destabilise security and there have been protests in South Korea itself, where three people were injured in clashes with police as the system was being delivered to a former golf course on Wednesday.
Tensions have been rising amid fears North Korea could be planning further missile or nuclear tests.
Speaking ahead of the classified briefing, Adm Harris said the advanced missile defence system would be ready within the coming days.
He said he believed that North Korea would try to attack the US as soon as it had the military capabilities.
"With every test Kim grows closer to his goal, which is using nuclear weapons on US cities," he told the House armed services committee in Washington DC.
"As [US President Donald] Trump and [Defence Secretary James] Mattis have said, all options are on the table," he added.
President Trump invited all 100 senators to the briefing, which took place in a building next to the White House.
Adm Harris said that the relationship between Mr Trump and Chinese President Xi Jinping remained "positive and encouraging".
"It is purely a defensive system. It is aimed north, not west," he said, adding: "It poses no threat to China."
His comments come after China launched its second aircraft carrier in the latest sign of its growing military strength.
The as-yet unnamed ship was transferred into the water in the north-eastern port of Dalian, state media said. It will reportedly be operational by 2020.
The heated rhetoric between the US and North Korea has intensified in recent weeks.
China's Foreign Minister, Wang Yi, has called for an end to US-South Korea war games and also a halt to North Korea's nuclear development in order to reduce tensions in the region.
Foreign ministry spokesman Geng Shuang spoke out against the Thaad deployment. He told reporters: "It helps in no way to achieve the denuclearisation of the peninsula and regional peace and stability."
The Trump administration has been urging China to rein in its ally, North Korea.
The Thaad system is designed to intercept and destroy short and medium-range ballistic missiles during their final phase of flight.
"South Korea and the United States have been working to secure an early operational capability of the Thaad system in response to North Korea's advancing nuclear and missile threat," South Korea's defence ministry earlier said in a statement.