China has built a potent military machine over the past 30 years but is struggling to develop advanced engines that would allow its warplanes to match Western fighters in combat, foreign and Chinese industry sources said.

The country’s engine technology lags that of United Technologies unit Pratt & Whitney, General Electric and Rolls-Royce, said Douglas Barrie, senior fellow for military aerospace at the International Institute for Strategic Studies in London.

China’s Defence Ministry, in a brief statement to Reuters, said there was a “definite gap” between Chinese military technology and some developed countries, adding Beijing would continue to strengthen its armed forces.

Western restrictions on arms exports to China prohibit the sale of Western engines for military use, forcing China to rely on homegrown designs or engines Russia has agreed to sell.

“Chinese engine-makers face a multitude of problems,” said Michael Raska, assistant professor in the Military Transformations Programme at Singapore’s S. Rajaratnam School of International Studies.