An airstrike on a funeral in Yemen has become the deadliest incident in the country's 19-month civil war - but who is fighting whom?
Who is doing the fighting?
Since March 2015, Yemen, the Arab world's poorest country, has endured a bloody conflict between Houthi rebels and Yemen's internationally-recognised government.
The Yemeni government, headed by President Abdrabbuh Mansour Had, has the support of the US, UK and France.
More importantly, a Saudi Arabia-led coalition - including the UAE, Bahrain, Kuwait, Jordan, Morocco, Qatar, Senegal and Sudan - are providing the government with military support.
Iran - a long-standing enemy of Saudi Araba - is said to tacitly support the Houthi rebels, although Iran denies this.
Additionally, so-called Islamic State has proclaimed several provinces in Yemen vowed to wage war against the Houthis, while al Qaeda also operates there.
The West regularly leads operations against Islamic State and al Qaedaoutposts in Yemen.
What is the background to the fighting?
The Houthis, a Shia-led political movement, emerged in the 1990s from northern Yemen.
Tension between the Houthis and central government grew throughout that decade, while fighting has been on and off since 2004 until it was ratcheted up last year.
Economic woes, political marginalisation, discrimination and a struggle for power have all contributed to the tensions.
Fighting was initially confined to the Houthis' northern stronghold, Saada province, but in September the rebels took control of Yemen's capital, Sanaa, and declared their own government, forcing President Hadi to flee.
The rebels then began pushing further south towards the country's second city, Aden.
How many casualties have there been?
More than 10,000 people are believed to have been killed in the conflict to date.
The conflict has caused a [humanitarian disaster](http://The humanitarian crisis facing Yemen is becoming clear) and a widespread shortage of essentials throughout the country.
According to the United Nations, more than 28 million of Yemen's people do not have enough to eat.
By January this year, the UN estimated more than 2.4 million Yemenis had been forced to flee their homes, with 120,000 seeking asylum in neighbouring countries such as Somalia and Djibouti.
Both Saudi airstrikes and Houthi rebels have been blamed for the killing of civilians.
Suicide bomb attacks by so-called Islamic State killed 126 in Sanaa early last year.
Peace talks between rebel and government parties in Kuwait failed in August.
Since then, the coalition has intensified airstrikes, despite criticism that the bombardments have been indiscriminate and could constitute war crimes.
The West has come under increasing pressure to stop selling arms to Saudi Arabia, while the US government has warned it will review its support for the coalition in the wake of Sanaa's funeral bombing.
Initial reports suggest that 140 people were killed and over 525 injured when an airstrike hit a hall packed with mourners.
"US security cooperation with Saudi Arabia is not a blank check," a National Security Council spokesman said.