Quoting a US military source, Aljazeera's correspondent in Baghdad said the ambush took place near al-Iskandariya on Monday morning.
A US spokesperson was not able to provide the names of those injured, but confirmed that other US occupation units in Baghdad had also come under attack.
Iraqi resistance fighters also launched a mortar attack on the presidential palace in the Adhamiyya district - a building used by US forces as military headquarters.
US military authorities would not comment on casualties or provide details.
Earlier on Monday, a roadside bomb wounded three civilian security contractors working in northern Iraq for the London-based firm Global Risk Strategies, the US military said.
The firm has about 1100 mercenaries on the ground in Iraq - mainly armed former Nepalese and Fijian soldiers.
The incident is the second attack against armed contractors in as many days in Mosul, a major city in northern Iraq.
The British Foreign Office said that a British mercenary was killed and three colleagues were wounded in a drive-by shooting Saturday in Mosul.
The four worked for ArmorGroup, a security firm with 1000 employees in Iraq protecting official buildings and companies.
Iraqi children play on the remains
of a contractor's car in Mosul
In a separate attack, a US security company confirmed that four of its employees two Americans and two Poles were killed Saturday in an ambush on the main road to Baghdad airport.
The company, Blackwater USA, also lost four employees in an ambush last March in Fallujah that triggered the bloody three-week siege of the restive Sunni Muslim city.
Al-Sistani speaks out
The attacks come as Iraq's top Shia cleric, Ayat Allah Ali al-Sistani, warned against any reference to the interim constitution in a UN Security Council resolution.
In a letter to the current Council president on Monday, al-Sistani rejected present attempts to include "the law of the Iraqi transitional government" within the new resolution of the Security Council a violation of the laws.
In a copy faxed to Aljazeera's bureau in Baghdad, the cleric insisted the 8 March law approved by the now-dissolved Iraqi Governing Council was "illegal and is rejected by most Iraqis".
"Any attempt to give it legitimacy by mentioning it in the resolution would be seen as contrary to the will of the Iraqi people and could have dangerous consequences."
Najaf-based Al-Sistani maintains that the "transition" law was established under occupation by a non-elected council and was directly influenced by the occupier.
"It sets restrictions on the national assembly which will be elected in 2005 and will write the constitution," he added.
The cleric has repeatedly slammed the basic law - due to remain in force until the end of the transition period and legislative elections planned for January 2005 - as an obstacle to a permanent constitution.