* Visit aimed at strengthening relations
* Angola is China's biggest African trading partner
LUANDA, Nov 19 (Reuters) - Chinese Vice President Xi Jinping began a visit to oil-rich Angola on Friday, saying he believed relations with his country's biggest African trading partner would be stepped up.
On the second leg of a four-nation tour, Xi will hold talks with Angolan President Eduardo dos Santos and said his visit was aimed at improving bilateral cooperation between the two countries, Xinhua news agency reported.
"I believe, thanks to the efforts from both sides, the Sino- Angola relations will continuously march into a higher level," Xinhua quoted Xi as saying.
Xi, touted as China's next president, arrived in Angola from South Africa after a three-day visit to Africa's biggest economy aimed at securing minerals China needs to fuel its rapid growth.
Angola is Africa's biggest oil producer after Nigeria and has had strong ties to China for decades.
China has given it $4.5 billion in loans since the end of a devastating 30-year civil war in 2002, China's ambassador to Angola, Zhang Bolun, said on Thursday.
Zhang said bilateral trade between China and Angola was expected to reach $20 billion this year, a big increase on 2009.
Angola's plans to reduce its dependence on oil could create further scope to improve relations, with China seeing potential in agriculture, service industries, infrastructure and renewable energy, he added.
Xi will visit Botswana, the world's biggest diamond producer, as the last stop on his African tour.
China's mineral diplomacy has been criticised for its secrecy and bolstering governments Western states accuse of human rights violations.
A senior World Bank executive said Chinese companies eyeing African mineral resources must stop making closed-door deals and become more transparent in their investments. (Writing by Marius Bosch; Editing by Ed Cropley)