Mattis spoke with the South Korean Defense Minister Han Minkoo Tuesday ahead of his inaugural trip as secretary of defense to visit Japan and South Korea, the US' closest Asian allies.
Han and Mattis expressed their commitment to proceed with the THAAD missile defense system deployment and the need for closer co-operation on policies on North Korea, according to a statement released by the South Korean Ministry of Defense.
The THAAD system is designed to take down incoming projectiles and has been cited as a way to potentially stop a nuclear attack from North Korea.
The visit comes after Donald Trump, when campaigning for the US presidency, caused concern in Asia by suggesting Seoul and Tokyo develop their own nuclear weapons. Trump also suggested the two countries either pay more for their own defense, or the US provide them with less security support.
Amid heightened tensions
The phone call between Mattis and Han comes amid analysis of new satellite imagery that suggests North Korea is restarting a plutonium reactor
Tensions are already high on the Korean Peninsula after North Korean leader Kim Jong-Un said his country was close to testing an intercontinental ballistic missile
(ICBM) -- the type of projectile that would be used to deliver a nuclear warhead -- in a televised New Year's Day address.
In the speech, Kim referred to North Korea as a "nuclear and military power in the east" and warned the US and its "vassal forces to stop nuclear threat and blackmail," according to state news agency KCNA.
In 2016 North Korea backed up its fiery rhetoric with two nuclear tests -- the country had only conducted three before in 2006, 2009 and 2013 -- and a handful of land and sea-based missile tests.
China trying to stop THAAD
China is concerned about THAAD at the strategic level, analysts suggest, with the country viewing the defense system as part of a broader US strategy to extend its military alliance network from Japan all the way down to the South China Sea.
"The Chinese see THAAD as part of a containment network that's not directed at North Korea, but them," John Delury, associate professor of international studies at Yonsei University in Seoul, told CNN.
Beijing is doing "everything in its power to prevent the deployment of the defense system," Delury added, including punitive measures on South Korean celebrities and companies.
Earlier in January, local Korean media reports suggested that everything from cosmetics to K-Pop stars had been hit hard by the South Korea-US alliance over THAAD.
When asked whether the cancellation of a concert by Korean musicians Sumi Jo and Kwun-woo Paik was owing to THAAD, during a press conference
in January, Chinese government spokeswoman, Hua Chunying denied the claims.
But Kim Hankwon, a national diplomatic academy professor in South Korea, told CNN that it was likely that the import ban on certain Korean products and cultural exports was related to THAAD.
He said that in the long term, however, China would avoid going overboard with their warning messages.
"If China implements stronger punitive measures towards South Korea, South Koreans will look more towards the US and show hostility to China," said Hankwon.