As they advance south, Yemen's Iran-linked Huthi militiamen are moving within striking distance of the strategic Bab al-Mandab strait, a vital corridor through which much of the world's maritime trade passes.
Only about 30 kilometres (20 miles) across at its narrowest point, the strait separates the Arabian Peninsula from east Africa and links the Red Sea with the Gulf of Aden.
Nearly 40 percent of global maritime trade is estimated to pass through the strait, much of it on its way to and from the Suez Canal.
As Yemen's Shiite Huthi militiamen have moved south after seizing the capital Sanaa last year, concern has been growing about their intentions for Bab al-Mandab.
The militia on Sunday took control of the airport in the key central city of Taez, tightening the noose on President Abedrabbo Mansour Hadi in his refuge in the southern city of Aden only about 180 kilometres (110 miles) away.
Hadi fled to Aden after escaping house arrest in Sanaa last month, and the country has increasingly been divided between the Huthi-controlled north and presidential loyalists in the south.
Security sources say Huthi forces have been dispatched from Taez to the port of Mocha, some 80 kilometres to the west.
From Mocha, a coastal road of around 100 kilometres leads to Bab al-Mandab.
If the militia does make a move to take control the strait, experts say, Yemen's crisis could quickly become a global problem.
The Huthis have been closely linked with Iran, which already overlooks another maritime chokepoint, the Strait of Hormuz linking the Gulf with the Arabian Sea.
If the militia takes control of Bab al-Mandab, "Iran would be the main winner," said Bassem al-Hakimi, a Yemeni political expert.
- 38% of maritime trade -
 He said such a move would give Tehran an additional "card to play in the negotiations over its nuclear programme" with world powers.
There is no doubt that the seizure of coastal areas on the strait would raise international concern.
Both the United States and France maintain a military presence on the other side of the strait in Djibouti, and for Egypt the strait is of crucial importance.
Egypt's ambassador to Yemen, Youssef al-Sharqawi, warned recently that threats to Bab al-Mandab would be a "red line" for Cairo.
"More than 38 percent of global maritime trade passes through the strait," he told reporters in Aden.
"The national security of Yemen is closely linked to the security of the Red Sea, the Gulf and Bab al-Mandab."
Israel has also raised concerns, with Prime Minister Benjamin Netanyahu warning of an Iranian threat to the strait in a speech to the US Congress earlier this month.
"Backed by Iran, Huthis are seizing control of Yemen, threatening the strategic straits at the mouth of the Red Sea. Along with the Straits of Hormuz, that would give Iran a second chokepoint on the world's oil supply," he said.
But some experts are sceptical about any danger to the strait posed by the Huthi advance.
"The whole thing is a total red herring," said Richard Dalton, a former British ambassador to Iran and associate fellow at the Chatham House think tank.
Dalton said there was no reason to believe that Tehran -- whose links with the Huthis he said have been exaggerated -- would want to complicate shipping through the strait.
"The Iranians are pro-free passage and they play by the rules. They want to be respected so it is unlikely that either Iran or the Huthi movement would seek to disrupt shipping there," he added.