* Air France-KLM posts record 2023 revenues but Q4 loss

* IAG positive on capacity for 2024

* Both see strong demand

* Jet fuel prices, geopolitical uncertainty, wage talks weigh

LONDON, Feb 29 (Reuters) -

British Airways owner IAG and Air France-KLM on Thursday gave a positive outlook for 2024 on the back of sustained travel demand, but the Franco-Dutch carrier's year-end troubles highlighted some of the challenges the sector faces.

European airlines are reporting strong summer bookings as they benefit from a continuing post-pandemic travel boom, but concerns over high jet fuel prices, geopolitical uncertainty and wage talks have clouded prospects.

Case in point: Air France-KLM swung to an unexpected loss in the last quarter of 2023, hit by higher costs and disruptions caused by conflict in the Middle East, sending its shares down 10% in early trade.

IAG, however, saw its shares rise following strong 2023 results and reassuring comments over capacity for 2024, amid fierce competition for new planes and problems at manufacturers.

IAG CEO Luis Gallego said the Middle East conflict had impacted mostly corporate demand in the last quarter of 2023 and the first quarter of 2024, but that was expected to recover.

Unlike other airlines, IAG said it was not concerned over capacity for the year to come and was not expecting delays in Boeing deliveries this year.

Gallego said that if the certification of Boeing's 737 MAX 10 was slowed down, IAG could convert to other variants. Boeing is grappling with quality issues and a regulatory crackdown following the blowout of a panel from a brand new Alaska Airlines MAX 9 during a flight on Jan. 5.

"For the time being we aren't worried. We are sure they'll fix the situation," Gallego told a press call.

Sophie Lund-Yates, lead equity analyst at Hargreaves Lansdown, welcomed IAG's drive to improve efficiency by reducing disruptions.

"These are all great targets, but the pace of delivery is far from guaranteed. It's crucial that BA gets this right," she added.

UNEXPECTED Q4 LOSS

In another example of possible clouds on the horizon, despite strong demand, analysts and investors have told Reuters that Lufthansa will likely miss its 2024 profit margin goal as the German airline seeks to agree new, higher pay deals to end prolonged strikes.

Meanwhile, Air France-KLM reported record revenues for 2023 and an operating profit of 1.7 billion euros ($1.8 billion), in line with expectations.

However, its fourth-quarter operating loss of 56 million euros was far short of analysts' consensus forecast for a profit of 88 million euros, according to a company poll.

Still, Air France-KLM was able to pay down 1.3 billion euros in debt, much of which was amassed during the pandemic travel shutdown, leaving outstanding net debt at 5 billion euros.

"We can be satisfied of our efforts to further strengthen our balance sheet and restore the Group's equity," Chief Executive Ben Smith said in a statement.

The group said it expected that costs will not go up as quickly as they did in 2023, and added that for the summer of 2024 its capacity would be close to 2019 levels.

Finance chief Steven Zaat said there was strong demand. "The problem is not selling the tickets. We can sell every seat we want," he said, adding the issue was having enough capacity.

Bernstein analysts said Air France-KLM's fourth quarter loss was impacted by one-off effects of disruption costs and the implementation of an employee shareholding plan.

"Importantly, real-terms cost reduction looks set to continue, supporting margin expansion ahead," they added.

($1 = 0.9230 euros)

(Writing by Ingrid Melander and Joanna Plucinska Editing by Mark Potter)