The United States also announced new sanctions on Friday in response to North Korea's latest missile launches this week, targeting a fuel procurement network that it said supports Pyongyang's weapons programs and its military.
South Korea's Joint Chiefs of Staff said the maritime drills with the USS Ronald Reagan Carrier Strike Group would continue into Saturday in waters off South Korea's east coast.
The drills come after North Korea fired an intermediate range ballistic missile over Japan on Tuesday, then launched a pair of ballistic missiles into the sea on Thursday and flew warplanes near the South Korean border.
"We will continue to strengthen our operational capabilities and readiness to respond to any provocations by North Korea," South Korea's Joint Chiefs of Staff said.
The U.S. strike group already participated in trilateral missile defence exercises with warships from Japan and South Korea this week.
Senior defence officials from Japan, South Korea and the United States discussed the latest developments in a call on Friday, condemning the North Korean launches and agreeing that recent trilateral maritime drills have improved their ability to respond to North Korea, South Korea's ministry of defence said.
In a meeting with South Korean Defence Minister Lee Jong-sup, Admiral John Aquilino, the commander of the U.S. Indo-Pacific Command, said North Korea's provocations seriously threaten regional security. South Korea's Defence Ministry quoted Aquilino as saying that his forces would actively support efforts to deter and respond to the North Korean threat.
Lee also held a meeting of senior officials to discuss the latest launches and to evaluate South Korea's "three-axis" defence system designed to counter North Korea's military threats, including war plans that call for preemptive strikes if necessary.
"He emphasised the need to deliver a clear message that nuclear and missile development will only create a more difficult situation for North Korea," the defence ministry said.
The rare bombing drill by at least eight North Korean fighter jets and four bombers on Thursday prompted South Korea to deploy 30 fighters on its side of the heavily fortified border.
On Thursday, North Korea condemned the United States for repositioning the aircraft carrier near the peninsula, saying it posed a serious threat to the stability of the situation.
North Korea's foreign ministry also criticised Washington for calling a United Nations Security Council meeting over the launches, saying the were a "just countermeasure" to the joint U.S.-South Korea drills.
Washington announced its new unilateral sanctions after accusing China and Russia on Wednesday of enabling North Korean leader Kim Jong Un by blocking attempts to strengthen U.N. Security Council sanctions targeting Pyongyang's nuclear weapons and ballistic missile programs.
South Korea's foreign ministry said the nuclear envoys of the United States, South Korea, and Japan held a call and vowed to redouble efforts to block North Korea's nuclear and missile financing through the seizure of cryptocurrencies, and to prevent attempts to evade sanctions such as illegal maritime transhipments.
Russia's envoy called new sanctions a "dead end" and China said it preferred to focus on constructive measures to reduce tensions.
Decades of U.S.-led sanctions have not stemmed North Korea's increasingly sophisticated missile and nuclear bomb programs, and North Korean leader Kim has shown no interest in returning to a failed path of diplomacy he pursued with former U.S. President Donald Trump.