WASHINGTON (REUTERS) - An Ohio man who trained with Islamic militants in Syria pleaded not guilty on Friday in a Columbus federal court to charges of supporting terrorism and making false statements, court documents said.

Abdirahman Sheik Mohamud, 23, a naturalised American, had been instructed by a Muslim cleric to return to the United States and carry out an act of terrorism, a federal indictment said.

It said Mohamud had told an associate that he wanted to go to a military base in Texas and "kill three or four American soldiers execution-style."

Before travelling from the United States to Syria, Mohamud had posted material on social media promoting symbols of the Islamic State militant group, the indictment said.

However, a law enforcement official told Reuters that once in Syria Mohamud and his brother, Aden, trained with the Nusra Front, the local affiliate of Al-Qaeda.

In Syria, he sent videos of himself to an unnamed person, the indictment said. In one video, he pointed to a gun in a holster on his hip ,and in another he stood in front of a white house with a black flag on it.

Mohamud's brother was killed fighting with the Nusra Front, according to the indictment.

Upon his return to the United States, it said, Mohamud told an unidentified person that while in Syria, he had been trained in shooting weapons, breaking into houses, using explosives and engaging in hand-to-hand combat.

According to the indictment, Mohamud "wanted to kill Americans, and specifically wanted to target armed forces, police officers or any uniformed individuals".

The indictment said Mohamud's plan was to attack a US military base but that he also had a backup plan to attack a prison.

Mohamud also waived a detention hearing on Friday, and will remain in custody, court documents said. His trial is set for June 22.