Federal authorities say they are investigating "potentially hazardous" letters containing "suspicious substances" that have been sent to congressional and state election offices around the US.

The FBI and the US Postal Inspection Service (USPIS) said they were trying to trace the "suspicious mailings", some of which were intercepted before they were received.

Authorities declined to say how many offices had been targeted, but several secretaries of state and congressional offices reported receiving the letters.

Coming just weeks before the US midterm elections, it echoes previous security scares involving suspicious mail.

"This FBI and the US Postal Inspection Service will continue to safeguard the public and secure elections for American citizens," the agencies said in a statement on Thursday.

The Oklahoma State Election Board and the Louisiana secretary of state both said this week that their offices had received letters containing a white powder.

The Louisiana office was evacuated after a staff member opened the package on Tuesday, but it has since reopened after law enforcement found no evidence of harmful substances.

Ohio Secretary of State Frank LaRose said his office was the intended recipient of a letter containing white powder, but the USPIS had intercepted it.

Senator Chris Murphy, a Connecticut Democrat, was also sent a letter containing a suspicious substance, but it was intercepted before reaching the office, his spokesman said.

The New York State Capitol in Albany and the Wyoming State Capitol in Cheyenne also received suspicious packages, according to emergency responder scanner feed reviewed by the BBC.

The suspicious mailings come just weeks before election day, which will determine the balance of power in Congress.

Ahead of the 2024 election, vote officials in 17 states received suspicious packages.

A year earlier, suspicious envelopes, some laced with fentanyl, were sent to election offices in at least five states.