South Korea's 20th National Assembly officially kicked off its first regular session on Thursday that will run through December, with parties likely to continue clashing over key issues including the deployment of an advanced US missile defense system in the country.

Political observers said the ruling and opposition parties are set to lock horns over core issues such as next year's budget and other policy-related matters with the next presidential election set for late 2017. 

The ruling Saenuri Party, as well as the Minjoo Party of Korea and the People's Party have been at odds over the Terminal High Altitude Area Defense system that Seoul agreed to host in July.

The conservative Saenuri claims the system is vital to tackle North Korea's developing missile provocations, while Minjoo and the People's Party stress Seoul should refrain from taking steps that would make Beijing uncomfortable.

Choo Mi-ae, newly-elected head of Minjoo, also implied she will make some progressive changes to the party's policies, compared to her neutral predecessor Kim Chong-in.

Following the latest development of the firing off of submarine launched ballistic missiles by Pyongyang, Saenuri is also anticipated to announce more aggressive measures to tackle the provocations, including building nuclear-powered submarines.

The scheduled hearings on financially troubled shipbuilders and shippers can potentially act as hurdles for the new session of parliament.

"The government's attitude towards the THAAD cannot win approval," National Assembly Speaker Chung Sye-kyun said during the opening speech of the session. "Regardless of the inevitability of the THAAD, there has been no communications from inside. This resulted in the division of the public opinion and confusion among the people."

"Although there needs to be an appropriate response to North Korea's reckless actions, we should try to avoid the current situation where Seoul and Pyongyang are taking extreme measures, Chung added.

Chung said the parliamentary session should focus on improving the livelihood of the people, while narrowing down economic gaps and overcoming inequality.

The National Assembly will hold audits on core diplomatic, security, social, and economy issues from Sept. 26 to Oct. 15. They will also finalize next year's budget plan by Dec. 2.

While the ruling Saenuri Party controls 129 seats, it failed to secure a majority in the 300-seat parliament.

Pundits said the inability of one party to pass laws may hold up key debates and legislative process. (Yonhap)