CAIRO, March 13 (Reuters) - Egypt's army blamed the outlawed Muslim Brotherhood for an attack on an army bus which killed one officer and wounded three others in the capital on Thursday, violence underscoring growing security threats to the military-backed government.

Masked gunmen had opened fire on the vehicle in central Cairo, security forces said.

"Masked armed men belonging to the terrorist Brotherhood targeted a bus of the armed forces ... which led to the martyrdom of the Warrant Officer Yusri Mahmoud Mohamed Hassan," the army spokesman said in a statement posted on Facebook.

Islamist militants are expanding their insurgency in Egypt where army chief Field Marshal Abdel Fattah al-Sisi, who overthrew Mohamed Mursi of the Muslim Brotherhood in July, is expected to announce he will run for president within days.

The most active group, Ansar Bayt al-Maqdis, has claimed responsibility for a series of high-profile attacks on senior security officials, including an assassination attempt on the interior minister last year.

A security crackdown has devastated the Brotherhood, driving Egypt's most organised political organisation underground. The Brotherhood, which the interim government declared a terrorist group in December, says it is committed to peaceful activism.

Most of its leaders are in prison and it denies carrying out attacks.

Militants based in the Sinai peninsula near the Israeli border are still a major security threat to the Arab world's biggest nation despite army offensives, including air strikes.

They have stepped up attacks on soldiers and policemen since Mursi's ouster, killing hundreds and spreading their campaign to Cairo and other cities. (Reporting by Yasmine Saleh and Shadia Nasralla; Writing By Noah Browning; Editing by Michael Georgy and Louise Ireland)