American troops have strengthened their positions around Baghdad in a drive to encircle the city, US commanders say.
The US army holds control of Baghdad's main airport
US forces are reported to have reached the River Tigris north of the city and to have established control of the western half of a circle around Baghdad with marines moving to extend it to the east.
Iraqi officials have acknowledged that US-led troops approached the southern outskirts of the city but say Republican Guard units repelled the advance.
The move to surround the capital came as coalition units made significant advances further south in the strategic cities of Karbala and Basra.
The US military also says it is investigating why one of its planes bombed a convoy of Kurdish fighters, US soldiers and western journalists in northern Iraq. At least 15 people were killed and 45 injured.
On Sunday evening a US military C-130 cargo plane became the first flight to land at Baghdad's international airport since US forces seized the area on Friday, US officials said.
Fierce fighting erupted in the western suburbs of the capital when units of the US 3rd Infantry encountered determined resistance from Iraqi troops.
The US soldiers fought their way north, killing or wounding hundreds of Iraqis and inflicting damage on their equipment, a US military official said.
The Iraqis lost 23 tanks, 22 artillery pieces and 30 armoured personnel carriers, according to a 3rd Infantry spokesman.
US commanders say there will be more incursions into the city to maintain pressure on Iraqi troops.
Iraqi authorities say they have strengthened defences on the streets of Baghdad and dispute US claims that coalition forces control the airport.
'Just about there'
US commanders say they will build on their gains so far to complete the encirclement of the capital.
"We're just about there," Colonel Will Grimsley of the US 3rd Infantry Division told Reuters news agency.
"Look at it from this point of view: 1st Brigade holds the airport and the west of Baghdad, the 2nd Brigade is securing the south, the 3rd Brigade is holding the north-west and the marines are in the north-east," he said.
On the edge: A curfew has been imposed on Baghdad
The coalition air campaign continued over the city, with six explosions hitting the southern outskirts early on Monday.
The BBC's Paul Wood in Baghdad said earlier that the air was being punctured by the sound of Iraqi rockets fired from the centre of the city - a counterpoint to the US warplanes coming in lower and more often than ever before.
Artillery explosions rocked the residential area of Zayouneh in east Baghdad on Sunday, near to an Iraqi Olympic Committee building of President Saddam Hussein's son Uday, Reuters said.
Mortar bombs are also reported to have landed in the Saadun area in the heart of Baghdad.
A heavy exchange of fire, including artillery, mortar, machine-gun and rocket blasts, could also be heard in southern Baghdad on Sunday evening, AFP news agency said.
US Central Command, based in Qatar, says a new air war strategy is being implemented over Baghdad, to provide direct cover for more incursions by US ground troops.
US commanders say they now control the city of Karbala, 80 kilometres (50 miles) south of Baghdad.
British troops launched a major assault on Basra
They said US troops killed about 400 Iraqi soldiers during two days of fierce house-to-house fighting in the city.
One US soldier was killed and eight others wounded in the battle, according to officials.
Further south, UK forces say they have secured "large parts" of Iraq's second city, Basra, after entering the city early on Sunday.
By Sunday evening most of the city, with the exception of the old town, was reported to be in British hands - although sporadic fighting was continuing.
The UK defence ministry said three British soldiers had died, although a spokesman said Iraqi opposition was patchy.
In other developments:
- Kurdish fighters and US forces push closer to the two main Iraqi-held cities in the north - Mosul and Kirkuk
- A Russian diplomatic convoy evacuating staff from Baghdad to Syria is caught in crossfire, with several people reportedly injured
- US President George W Bush and UK Prime Minister Tony Blair are scheduled to discuss the progress of the war when they meet in Northern Ireland on Monday