Prime Minister Jacinda Ardern has expressed New Zealand's sympathies to those affected by shootings in El Paso, Texas, over the weekend.

At least 20 people were killed and 26 injured in a Walmart store in El Paso on Sunday.

Less than 24 hours later nine died and 16 were wounded in Dayton, Ohio.

"New Zealand's condolences go out to those killed and injured in the El Paso shooting and their loved ones," Ardern said before news of the Ohio shootings broke.

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"I know no one in New Zealand would want the terrorist act that occurred here to inspire anything other than a sense of unity against acts of hatred, violence and terrorism."

The El Paso death toll could rise, officials said, noting that some of the dozens of people injured were in critical condition.

President Donald Trump tweeted the "reports are very bad, many killed," adding that he had spoken to Texas Gov. Greg Abbott to "pledge total support of Federal Government."

Ardern said there had been no confirmation the El Paso shooting was connected to the Christchurch shooting in March where 51 people died.

Prime Minister Jacinda Ardern. Photo / Mark Mitchell
Prime Minister Jacinda Ardern. Photo / Mark Mitchell

However, the FBI is investigating claims the alleged gunman posted a manifesto citing the mosque attacks not long before the attacks.

A manifesto containing white supremacist content was apparently posted online before the attack, on the same forum used by the accused Christchurch shooter.

The FBI is now examining the manifesto, Texas media reported.

The alleged shooter - named as Patrick Crusius, who just turned 21, of Dallas - is in custody. It is understood no shots were fired by police during his arrest.

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In the El Paso manifesto, the author claims to support the actions of the alleged Christchurch shooter, also supporting sentiments expressed in the manifesto.

The document claimed similar white supremacist reasons for the shooting. The author said they had spent less than a month preparing and listed the firearms they planned to use.

In the Dayton shooting, reported by AP at 9pm NZT, the suspected shooter was also dead.

People gather in Juarez, Mexico, in a vigil for the three Mexican nationals who were killed in an El Paso shopping-complex shooting. Photo / AP
People gather in Juarez, Mexico, in a vigil for the three Mexican nationals who were killed in an El Paso shopping-complex shooting. Photo / AP

Dayton police tweeted an active shooter situation began in the Oregon District at 1am, but officers nearby were able to "put an end to it quickly".

At least 16 others were taken to local hospitals with injuries, police said.

The suspected Dayton shooter has not been named, AP also reported. Police have not said where in the district the shooting took place.The FBI is assisting with the investigation.

Miami Valley Hospital spokeswoman Terrea Little said 16 victims have been received at the hospital, but she couldn't confirm their conditions.

Kettering Health Network spokeswoman Elizabeth Long said multiple victims from a shooting had been brought to system hospitals, but didn't have details on how many.

With a population of about 140,000, Dayton is in western Ohio, around 90km northeast of Cincinnati, 120km west of Columbus and 195km east of Indianapolis.

On July 28, a 19-year-old shot and killed three people, including two children, at the Gilroy Garlic Festival in Northern California.

The shooting in Dayton the 22nd mass killing in the United States this year. The first 20 mass killings in the US in 2019 claimed 96 lives.

Additional reporting Associated Press, Washington Post.