In an interview with Reuters, Trump said, "I informed South Korea it would be appropriate if they paid. It's a billion-dollar system."

Seoul officials have said that under a bilateral arrangement, Washington is financially responsible for the deployment, operation and maintenance of the THAAD unit, with Seoul just providing the land to host it.

Earlier this week, US forces here brought THAAD's radar system and other key components to Seongju, 296 kilometers southeast of Seoul, without prior notice, triggering protests from local residents concerned that their hometown could be a potential military target.

The liberal Democratic Party said that as Trump brought up the issue of financing the deployment, the need for discussions between the next Seoul government and the Trump administration has increased.

"The next government will have to discuss (with the Trump government) the diplomatic and economic dimensions of the THAAD deployment," Youn Kwan-suk, a party spokesman, told reporters.

Presidential front-runner Moon Jae-in of the party has said that the deployment must be deferred to allow the next government to make a decision on the crucial security issue.

The conservative Liberty Korea Party claimed that Trump made the remarks out of concern about the possible election of a "leftist" leader in the May 9 vote.

"(Trump's remarks) are not unrelated to the domestic political situation here," Kim Myung-yeon, a spokesman of the party, told reporters.

"There will be no concerns if the strong right-wing government led by Hong Joon-pyo, takes power," he added.

The center-left People's Party pressured the Seoul government to clarify whether it had a backroom deal with Washington over THAAD.

"If there had been any backroom agreement, this is tantamount to lying to the public, and the (process of securing) parliamentary consent (for THAAD) must precede its installation," Son Kum-ju, a party spokesman, said in a commentary.

The minor conservative Bareun Party said that Trump's remarks veered off from the existing agreement between Seoul and Washington over the deployment.

"If we have to purchase a THAAD system on our own, then we have to shoulder the cost of the deployment ... But as for this deployment (brought on by the US), there has already been an agreement (that the US is to pay for its deployment and operation)," Ji Sang-wuk, the party's campaign spokesman, said in a commentary.

In a separate statement, Yoo Seong-min, the party's presidential candidate, stressed that based on bilateral cooperation, the two sides can work together to find a solution to the issue.

The angry reaction came from Sim Sang-jeung, the standard-bearer of the progressive Justice Party.

"(I call on the US to) take THAAD back (to your territory)," Sim said on the stump in central Seoul.

"(The US) has pushed for the surprise installation of THAAD without public consent here and is now pressuring (Seoul) to purchase it ... The Republic of Korea does not need THAAD for the sake of the US (interests)," she added. (Yonhap)