LONDON (Reuters) - Britain's army is under pressure in Iraq and ramping up operations in Afghanistan this year, but if the situation on the ground does not improve fast Washington may soon find that its closest ally is stretched too tight.

Word of the 100th British death in Iraq arrived on Tuesday just days after the announcement of an ambitious new mission to southern Afghanistan that will see Britain's forces there increase from just 1,000 now to a peak of 5,700.

"When you spend your holidays from Afghanistan in Iraq, that is where you stretch your army to breaking point," said Tim Ripley, who writes for Jane's Defense Weekly.

"You're talking about them coming back from Iraq and going straight to Afghanistan and then going back again. And that puts a real strain on whether these people want to stay in the army."

British Prime Minister Tony Blair said there would be no turning back either in Iraq or Afghanistan.

"Our response (to the 100th death in Iraq) has got to be not to walk away from the situation but to redouble our efforts to make sure the people of Afghanistan and Iraq achieve the democracy they want," he told Reuters Television.

All told, Britain is in line to peak at some 14,000 troops deployed to the two countries by the middle of this year.

That would be easily manageable for a short time, military experts say -- Britain sent 45,000 troops to the Gulf for a few months during the 2003 invasion of Iraq. But the strain would start to show by the end of this year if the missions run on.

MORE THAN JUST NUMBERS

Long-term missions tie up three times the number of soldiers deployed, since for each unit in the field one will have just returned and another must be training to take its place.

"You've got to multiply those numbers by three to get the effect on the army," said Charles Heyman, editor of the annual handbook Armed Forces of the UK. "You add the two (missions) together and you've got almost half the trained strength of the British army committed."

More than total numbers is the strain on key "low density, high demand assets" like air transport, logistics and the specialized high-mobility units needed for a remote, landlocked country like Afghanistan, said Ripley.

"It focuses on a very narrow slice of the army which has really been put under pressure. When will they get a respite?"

IRAQ WITHDRAWAL HELD UP

When the plans for an expanded role in Afghanistan were being drawn up last year, British officials were describing a rapid draw-down of their force in Iraq. That now seems much further away than it did just a few months ago.

A memo signed by British Defense Secretary John Reid and leaked last July described Britain drawing its Iraq force down from 8,500 to just 3,000 by the middle of 2006.

But Brigadier Patrick Marriott, commander of the main UK force in Iraq, said last month he did not think any British withdrawals could even be considered until after local elections there in the next few months.

In both Afghanistan and Iraq the dangers for British troops have increased substantially since the middle of last year.

The mainly Shi'ite southern area of Iraq patrolled by Britain was once seen as one of the safest parts of the country.

But British forces have been caught up in quarrels between rival Shi'ite factions, and guerrillas have deployed new, deadlier forms of roadside bombs forcing British forces in many areas to remain confined their bases.

Marriott said his troops spend 60 percent of their effort just defending themselves, far more than just a few months ago, limiting their ability to train Iraqis to take over their role.

In Afghanistan, the British troops will join an expanding NATO peacekeeping force that is not supposed to hunt down the Taliban -- still a job for U.S. troops outside NATO command.

But Taliban attacks have become more frequent, and the guerrillas have begun carrying out suicide bombings in the last few months, a tactic that is new to the area. British commanders say their forces will have "robust" orders to allow them to strike back when they are attacked, and they expect a fight.