Taiwanese prosecutors said Wednesday they were investigating five people over accusations they had arranged free trips to China for dozens of voters in a bid to "influence" next month's elections.

Taiwan goes to polls on 13 January to elect a new president and a new parliament in a contest that has been dominated by relations with China. 

Relations have plunged in recent years as China has stepped up pressure on self-ruled Taiwan, which it regards as part of its territory to be seized one day, by force if necessary.

The five people under investigation were among 35 people questioned by prosecutors on Tuesday on suspicion of violating election and anti-infiltration laws.

Prosecutors from Kaohsiung in Taiwan's south said in a statement a man, identified only by his family name of Chou, and four others had been "in contact and received instructions" from Chinese officials in charge of Taiwan affairs to "invite influential local people" to China.

They alleged the five had arranged trips to Hunan province last month for 60 people from Kaohsiung and Taitung, another southern city, and offered "unfair benefits" that included free meals, accommodation and transportation.

In return, Chou "asked for their support of candidates from specific political parties and with specific political leanings in an attempt to influence voters' voting intentions and the election results", the prosecutors from Kaohsiung's Ciaotou district said.

The candidates and parties were not identified.

The Ciaotou prosecutors also questioned last month another 22 people, including two who were alleged to have arranged at least five trips to China for an unspecified number of voters subsidised by Chinese authorities.

Taiwanese government officials have warned that China could try to influence the island's elections. 

Beijing has blasted the ruling Democratic Progressive Party's candidate Lai Ching-te and his running mate Hsiao Bi-khim as an "independence duo" for their refusal to acknowledge its claim on Taiwan. 

The main opposition Kuomintang party's candidate Hou Yu-ih and third-party candidate Ko Wen-je, who have both pledged warmer ties with China, are trailing the DPP in opinion polls.