Trump also asked all Americans to wear face coverings, saying “they’ll have an effect” and show “patriotism”, the BBC reported on Tuesday.
He was not wearing a mask at the briefing, but has previously downplayed such personal protective equipment.
The president’s aides have reportedly asked him to adopt a new tone as virus caseloads spike across the US.
The daily White House news conferences ended soon after Trump suggested in April during freewheeling remarks from the podium that the virus might be treated by injecting disinfectant into people.
In his first White House coronavirus briefing for months on Tuesday, a more scripted president echoed what public health officials on his pandemic task force have been saying as he warned: “It will probably unfortunately get worse before it gets better.”
He added: “We’re asking everybody that when you are not able to socially distance, wear a mask, get a mask.
“Whether you like the mask or not, they have an impact, they’ll have an effect and we need everything we can get.”
The president is facing an uphill climb to re-election in November against Democratic challenger, Joe Biden, according to opinion polls.
Trump appeared on Tuesday without the medical experts who used to address the briefings. He kept his remarks brief and focused, avoiding sparring with reporters who asked a few questions.
He continued: “We’re asking Americans to use masks, socially distance and employ vigorous hygiene – wash your hands every chance you get, while sheltering high risk populations.
“We are imploring young Americans to avoid packed bars and other crowded indoor gatherings. Be safe and be smart.”
Trump has been reluctant to wear a mask himself in front of the media, claiming that some people only wore such face coverings as a political statement against him. The press pictured him recently wearing a mask for the first time as he visited a military hospital.