The Horn of Africa nation has been plagued by conflict since clan warlords overthrew a dictator in 1991. This was followed by fighting between rival clans and an Islamist insurgency.
Al Qaeda-aligned Al Shabaab, which seeks to topple the UN-backed government, often launches attacks in Mogadishu and across the country. It did not immediately claim responsibility for Sunday's attack and could not be reached for comment.
The Somali government could also not be reached for comment.
Residents living near the airport told Reuters several mortar bombs were fired at the compound. The diplomatic source said six people were injured after three mortar bombs were fired.
"I am appalled by this blatant act of terrorism against our personnel...Thankfully, the majority of our staff is unhurt," UN envoy James Swan said in a statement.
Al Shabaab has been trying for years to oust the central government and implement its strict version of Islamic law. It was driven out of the capital in 2011 but maintains a foothold in some regions including South West.
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