DAVOS, SWITZERLAND (REUTERS) - US President Donald Trump touted the success of the US economy in Davos on Tuesday (Jan 21), dismissing "perennial prophets of doom" on climate change to an audience that included Greta Thunberg.

With his impeachment trial set to begin in Washington, Trump largely shied away from environmental issues, which are top of the agenda at the gathering in a Swiss ski resort of business leaders for the annual meeting of the World Economic Forum.

Trump did not refer directly to teenage Swedish activist Thunberg, who responded to his speech by referring to "empty words and promises" from world leaders.

"You say children shouldn't worry ... don't be so pessimistic and then, nothing, silence," star act Thunberg said in her latest high-profile tit-for-tat with the US president.

Trump said the US would join an initiative to plant a trillion trees, but spent several minutes of his speech hailing the economic importance of the oil and gas industries.

After days of uncertainty over whether Trump would actually attend the meeting in Davos, especially considering his impeachment trial in the US is under way, the US president landed in the Swiss Alps on Tuesday.

It was the second time Trump has taken the stage at the WEF meeting. Two years ago, he urged companies to invest in America after passing the first tax cuts to encourage business spending.

This year, he stuck to the theme. In a wide-ranging address pitched to appeal to the Davos crowd, touting the achievements of his administration despite his unorthodox approach, Trump picked up on some of the themes he voiced in 2018.

He thanked overseas companies for investing in the United States and said the US was on far better economic standing than he had imagined when he took office three years ago.

"The time for scepticism is over," Trump said as he invited more foreign money.

"To every business looking for a place to succeed...there is no better place than the US," he added.

He told a packed auditorium that trade deals struck this month with China and Mexico represented a model and took his biggest swipe yet at the Federal Reserve, whose policies he says are holding back the US economy.

"The Fed raised rates too quickly and has lowered them too slowly," Trump said of the Federal Reserve, taking aim at the central bank's policy decisions.

A TRILLION TREES

Thunberg, one of many young people speaking at Davos this year, left Trump's address flanked by security and pursued by camera crews and reporters.

Trump later told reporters: "I'm a very big believer in the environment. I want the cleanest water and the cleanest air".

However, Nobel economist Joseph Stiglitz criticised the president's swipe at climate activists.

"As if what we are seeing with our eyes are not there. It's astounding," Stiglitz said.

In Washington, the impeachment trial begins in earnest in the US Senate after the Republican president was formally charged by the Democratic-controlled House of Representatives in December with "high crimes and misdemeanors".

Trump, who is expected to be acquitted by the Republican-controlled Senate, says he is innocent of the charges.