Two British soldiers missing in southern Iraq could be hiding, politicians and military leaders have said.
British troops have been moving through southern Iraq
The two went missing in an attack on a convoy near the southern city of Basra on Sunday and search-and-rescue teams are trying to find the men.
"We like to think they have dispersed and taken cover and are looking to clear their path and return to their unit," said British forces spokesman Group Captain Al Lockwood.
He said the convoy of vehicles had dispersed when it came under small arms fire and when they regrouped the pair's vehicle was empty.
"Every effort is being made to find them," Defence Secretary Geoff Hoon told a news briefing.
Elsewhere British troops have had a morning of "extremely heavy fighting", said the BBC's Caroline Wyatt at British forces HQ in northern Kuwait.
They have come under artillery, sniper and small arms fire as they try to secure the southern port towns of Umm Qasr and Basra in southern Iraq.
The campaign is proceeding according to our strategic plan
UK defence minister Geoff Hoon
But Mr Hoon insisted the war was "progressing well" despite some setbacks in the five days since it began.
"The campaign is proceeding according to our strategic plan," the defence secretary said.
He said southern Iraq was "broadly under the control" of coalition forces, who were also making "steady" progress northwards, while targeted bombing continued.
Marines were drafted in to Umm Qasr on Monday, after coalition forces trying to hold it continued to meet resistance.
Mr Hoon said he hoped it would be secured and the waters cleared of mines and opened to shipping within a few days, to allow humanitarian aid through.
UK forces spokesman Colonel Chris Vernon said UK troops, about 5km to the west of Basra, had exchanged artillery fire with Iraqi forces, in response to an attack.
He said there were two types of enemy in Basra - both regular army troops and about 1,000 "pretty determined" irregular militia, who often wore civilian clothes.
There are concerns for the humanitarian situation within the city because parts of it have been without water or electricity for at least two days.
Colonel Vernon said coalition forces wanted to bring humanitarian aid into Basra, but would only do so "when the conditions are right and on our terms".
The Rumeila oil fields west of Basra, which had on Friday been pronounced "safe" by British and US troops taking them, were pronounced "unsafe" by US officials on Monday morning.
BBC correspondent Tim Franks, reporting from the Iraqi border, said all British units were reporting "niggling, sniping attacks".
"These are coming from what are characterised as regime zealots and militia, using guns and rocket-propelled grenades."
Group Captain Lockwood said there remained "minor pockets of resistance", but UK troops were not being hampered by them and the regular Iraqi army continued to surrender.
He said: "We're encountering them, if necessary we're going round them. They're not impeding our advance at all, and as necessary we will go back and deal with them.
"We're making progress, we're on our timeline."
UK Prime Minister Tony Blair is to brief MPs in the House of Commons on the progress of the war on Monday afternoon.
American commanders said Sunday was their toughest day of fighting yet.
US Marines suffered up to 10 dead and 14 wounded in heavy fighting with Iraqi forces in and around the city of Nasiriya - north of Basra and Umm Qasr.
And two RAF crew members were confirmed killed when their Tornado GR4 was shot down by a US Patriot missile.
ITV news reporter Terry Lloyd and two colleagues are also believed dead after their jeeps were hit by friendly fire.