Security forces in Kismayu are blocking all entry to the southern Somali port city ahead of contentious local elections.
The authorities ordered the closure of air, land and sea borders until Friday - a day after the vote.
Before the lockdown, the head of the African Union (AU) force, an Ethiopian commander, was supposed to arrive in the city to resolve a dispute between Kenyan and Ethiopian troops over the poll.
Both Ethiopia and Kenya are part of the more than 20,000-strong AU force, first deployed in 2007 to try and help stabilise the country beset by decades of unrest.
The UN and government, backed by Ethiopia, want the Jubbaland provincial elections postponed, but the regional authorities, backed by Kenya, want them to go ahead.
The current leader of Jubbaland, Ahmed Mohamed Islam known by his nickname “Madobe”, is seeking re-election.
His Ras Kamboni militia seized Kismayo from the control of Islamist Al-Shabab militants in 2012 - with the backing of the Kenyan army.
But the UN-backed government is no fan of Madobe and sees him as an obstacle in its attempt to stamp authority on the country, BBC Monitoring reports.
Madobe fears the government may use Ethiopian troops to stop the vote – which observers say is the likely reason for the blockade.
According to the Reuters news agency, the stakes for all parties are high as Jubbaland is one of Somalia's more stable regions with lush farmland – and the port is a lucrative source of income.