An owner of a shop near the checkpoint, Safaa Abud Abbas, said a rocket-propelled grenade was fired at the US checkpoint by two people in a car, but missed the soldiers. The Americans retaliated by returning fire.
Medical sources at Samarra hospital told AFP two men, aged 25 and 62 had been shot dead, and that two teenagers, aged 13 and 14, were wounded.
A US military spokesman could not immediately confirm the deaths, but said they were being investigated.
In Tikrit, which like Samarra lies within an insurgency-dogged area, American forces detained six Iraqis, including two senior members of a network of Saddam Hussein loyalists in an overnight raid in the ousted leader's hometown.
Lieutenant Colonel Steve Russell said some of the detainees were involved in financing, organising and sometimes participating in anti-occupation attacks, including on aircraft.
Meanwhile, in northern Iraq, Kurdish security forces have arrested a suspected member of the Islamist group Ansar al-Islam as he tried to flee Kurdish-controlled northern Iraq, a political official said on Friday.
The source said the man had not been arrested over Sunday's twin suicide bombings in the Kurdish town of Arbil that killed at least 105 people.
Ansar al-Islam, which controlled an enclave of northeast Iraq before being crushed by US forces at the end of March, is suspected of links to Usama bin Ladin's Al-Qaida network.
The little-known Ansar al-Sunna group claimed responsibility for the attack in Arbil, while expressing sympathies with the "brothers" of Ansar al-Islam.