BEIJING, April 30 (Reuters) - China's manufacturing activity expanded at a slower pace in April, an official factory survey showed on Tuesday, suggesting some loss of momentum for the sprawling sector at the start of the second quarter.

The National Bureau of Statistics (NBS) manufacturing purchasing managers' index (PMI) dropped to 50.4 in April from 50.8 in March, above the 50-mark separating growth from contraction and just ahead of a median forecast of 50.3 in a Reuters poll.

The data underlines the challenges facing policymakers even though solid first quarter GDP growth has reduced some of the urgency to ramp up stimulus measures.

New export orders grew at a much slower rate, while employment continued to shrink, the NBS PMI data showed.

Another private Caixin factory survey, also released on Tuesday, showed the manufacturing activity grew more quickly as new export orders rose.

Investors expect Chinese authorities to launch more stimulus to support the economy and are waiting for the Politburo meeting, which is expected to focus on economic affairs this month. The Politburo is expected to hold its monthly meeting by the end of the day.

With the U.S. Federal Reserve and other developed economies in no hurry to cut interest rates, China may face a longer period of tepid external demand. Adding to the challenges, Beijing continues to contend with trade barriers as the U.S. accuses China of exporting its industrial overcapacity.

Officials this year underscored the need for economic development based on innovation in advanced sectors.

However, analysts said the country's immediate problem centres around a prolonged property downturn and ballooning local government debt, which have dented household and investor confidence in a blow to the economic outlook.

While stronger-than-expected first quarter economic growth provided a welcome impetus for the rest of the year, weakness in key areas such as March retail sales, industrial profits and property investment underline the challenge for China in sparking a broad revival in demand.

China has set a GDP growth target of around 5.0% for 2024, a goal analysts have described as ambitious. (Reporting by Ellen Zhang and Ryan Woo Editing by Shri Navaratnam)