A Russian military transport plane has taken off from an airfield outside Moscow and headed for the United States with a load of medical equipment and masks to help Washington fight coronavirus, Russian state TV reports.

President Vladimir Putin offered Russian help in a phone conversation with President Donald Trump on Monday, when the two leaders discussed how best to respond to the virus.

The flight, which was organised by the Russian Defence Ministry, is likely to be unpopular with some critics of Trump who have urged him to keep his distance from Putin and who argue that Moscow uses such aid as a geopolitical and propaganda tool to advance its influence, something the Kremlin denies.

"Trump gratefully accepted this humanitarian aid," Kremlin spokesman Dmitry Peskov was cited as saying by the Interfax news agency on Tuesday night.

Trump himself spoke enthusiastically about the Russian help after his call with Putin.

Russia's Rossiya 24 channel on Wednesday morning showed the plane taking off from a military air base outside Moscow in darkness. Its cargo hold was filled with cardboard boxes and other packages.

Confirmed US cases have surged to 187,000 and nearly 3900 people have already died there from COVID-19, the illness caused by the coronavirus.

In Russia, where some doctors have questioned the accuracy of official data, the official tally of confirmed cases is 2337 cases with 17 deaths.

Relations between Moscow and Washington have been strained in recent years by everything from Syria to Ukraine to election interference, something Russia denies.

Peskov, Putin's spokesman, said Moscow hoped the United States might also be able to provide medical help to Russia if necessary when the time came.

"It is important to note that when offering assistance to US colleagues, the president (Putin) assumes that when US manufacturers of medical equipment and materials gain momentum, they will also be able to reciprocate if necessary," Peskov was cited as saying.

Russia has also used its military to send planeloads of aid to Italy to combat the spread of coronavirus, exposing the European Union's failure to provide swift help to a member in crisis and handing Putin a publicity coup at home and abroad.