Boston Mayor Marty Walsh said Thursday that the city of Boston has joined a lawsuit against the Trump administration over new policies that would impose new and heftier fees on immigrants seeking to gain U.S. citizenship.

Earlier in the day, Walsh said, the city filed an amicus brief supporting Project Citizen, a nonprofit organization that advocates for immigrant access to citizenship, in suing the U.S. Department of Homeland Security. The department's new policies would nearly double the cost of applying for citizenship — from $640 to $1,160 — and would impose a new fee for asylum applicants.

“Think about who the asylum seekers are,” Walsh said. “Someone fleeing persecution, desperate for the freedom of the America promise. They’ve already paid a heavy price.”

Noting his own mother, who was born in Ireland, became a U.S. citizen, Walsh called the new citizenship application fees another “betrayal of what this country is all about.”

“American citizenship is not about what you believe or what you can pay,” Walsh said. “If you qualify to be a citizen, financial barriers should not be what stands in your way.”

“We are calling on the courts to stop this new rule because it harms our city and it goes against the values that America was founded on,” Walsh said.

Meanwhile, the mayor gave his usual updates on the city’s efforts to battle COVID-19.

While Boston continues to see relatively low positive test rates, one measure of prevalence of the virus, there has been an uptick in recent days. The citywide positive test rate for the week ending Sept. 12 was 2.8%, up from roughly 1.5% the week before.

Walsh said part of the increase may be attributable to a significant drop in the number of tests administered over the Labor Day holiday, but noted that the neighborhoods of Allston-Brighton, Dorchester, Hyde Park, Roslindale and West Roxbury have seen positive test rates rise to between roughly 4 and 5 percent.

“Any increase we see in numbers is concerning and we’re looking at these numbers very closely,” Walsh said.

Just under half of all new cases in the past two weeks were among people under 30 years old, Walsh said.

“We understand that young people are active and social, but we also need you to be cautious and be careful,” Walsh said, asking young people to maintain social distancing and avoid in-person social gatherings.

The news was better in East Boston, where Walsh said the positive test rate has dropped from over 11% in recent weeks to just over 5%.

Walsh also announced that CVS will be offering drive-through testing at three new locations, in Roslindale, Brighton and Allston.