Sheriffs in a dozen Washington counties are refusing to enforce the state’s new restrictions on the sale of semi-automatic rifles.

A state-wide initiative approved by voters in November raised the minimum age at which a person could purchase such weapons from 18 to 21, while adding expanded background checks and gun storage requirements.

The measures were seen as one of the more comprehensive of a series of gun-control reforms enacted in the US in the wake of the school shooting in Parkland, Florida, last year.

However, the new rules have been challenged in a lawsuit filed in federal court by the National Rifle Association (NRA) and the Second Amendment Foundation.

The advocacy groups claim the new purchasing requirements violate the right to bear arms and stray into regulation of interstate commerce, which falls under the purview of the federal, not state government.

Florida shooting – in pictures Show all 10 1 /10 Florida shooting – in pictures Florida shooting – in pictures Police arrest a suspect in connection with the shooting at the Marjory Stoneman Douglas High School in Parkland, Florida Reuters Florida shooting – in pictures Parents wait for news after reports of a shooting at Marjory Stoneman Douglas High School in Parkland, Florida AP Florida shooting – in pictures Anxious family members wait for news of students AP Photo/Wilfredo Lee Florida shooting – in pictures Students being evacuated from the school AP Florida shooting – in pictures Students being evacuated from the school Getty Florida shooting – in pictures People gather waiting for word from students AP Florida shooting – in pictures Parents waiting for news on their children AP Florida shooting – in pictures People gather at a hotel where students were taken after the shooting Getty Florida shooting – in pictures Florida Governor Rick Scott speaks to the media as he visits Marjory Stoneman Douglas High School following the shooting AFP/Getty Florida shooting – in pictures Dr. Igor Nichiporenko, Medical Director Trauma, left, and Dr. Evan Boyer, Medical Director, Emergency Services, speak about treating victims and the suspect at a press conference outside Broward Health North hospital AP

Sheriffs in 12 mostly rural Washington counties, along with the police chief of the small town of Republic, have said they will not enforce the new laws until the issue is settled by the courts.

“I swore an oath to defend our citizens and their constitutionally protected rights,” Grant County Sheriff Tom Jones said. “I do not believe the popular vote overrules that.”

Supporters of the initiative say they are disappointed, but noted the sheriffs have no role in enforcing the new restrictions until 1 July, when the expanded background checks take effect.

The provision brings vetting for semi-automatic rifle and other gun purchases in line with the existing process for buying pistols.

“The political grandstanding is disheartening,” said Renee Hopkins, chief executive of the Alliance for Gun Responsibility, which pushed for the new legislation.

“If they do not [run the background checks], we will have a huge problem.”

Initiative 1639 was passed by about 60 percent of Washington voters, nine months after a gunman killed 17 people at Florida’s Marjory Stoneman Douglas High School.

The shooting sparked a shift in the country’s political debate surrounding gun control and as a result some states introduced measures such as waiting periods and the outlawing of high-capacity magazines.

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Nine states have approved laws that allow the temporary confiscation of weapons from people deemed a safety risk, with several more likely to follow.

The lawsuit filed by the NRA and the Second Amendment Foundation does not challenge new restrictions on enhanced background checks or training requirements, only the rules banning the sale of semi-automatic weapons to those under 21.

However, the groups have asked the court to block the entire piece of legislation until it can be determined if other provisions contained within it can be separated out from the procedures they wish to block.

The state has asked the judge to dismiss the case.