China to keep opening markets: Li

China has imposed retaliatory tariffs on $US34 billion of American goods as a trade war begins

China will stick to the path of opening its markets and other reforms that has lifted its growth, Premier Li Keqiang says, a day after the Washington and Beijing slapped tariffs on $US34 billion ($A46 billion) worth of each others' imports.

China will open its door wider to foreign products as free trade needs to be firmly upheld to ensure sustained global economic growth, Li told a summit of eastern European leaders in Sofia on Saturday.

"For foreign products which meet Chinese consumer needs, we would open the door wider... We would lower overall import tariffs to the Chinese market," he said through an interpreter, without going into details.

Li said economic reform had played a critical role in China's growth, and that the fundamentals underpinning it remained unchanged.

"Opening up has been a key driver of China's reform agenda so we will continue to open wider to the world, including widening market access for foreign investors," he said.

Beijing earlier accused Washington of triggering the "largest-scale trade war". US duties on a range of Chinese imported goods took effect on Friday and were immediately countered by measures from China.

Neither side gave any sign of willingness to start talks aimed at a reaching a truce, though Li said on Friday that a trade war was never a solution and no one would gain from it.

The trade dispute has roiled share, currency and commodity markets in recent weeks.

President Donald Trump is already threatening additional rounds of tariffs, possibly targeting more than $US500 billion ($A673 billion) worth of Chinese goods, about the total amount of US imports from China last year.

China lodged a case with the World Trade Organisation against the US, its commerce ministry said on Friday.

China's tariff list is heavy on agricultural goods such as soybeans, sorghum and cotton, threatening US farmers in states that backed Trump in the 2016 US election, such as Texas and Iowa.