If 2019 started well for Martin Griffiths it is ending less positively. His job as the UN secretary general's special envoy for Yemen was to build momentum towards ending the war. Instead, he finds himself in the middle of a regional power struggle that could result in a wider Middle Eastern conflict involving the United States, Iran and Saudi Arabia.<br/> Mr Griffiths was instrumental in brokering a ceasefire in Hodeidah, Yemen's main Red Sea port, in December last year. But the peace talks between Yemen's government and Houthi rebels that led to that agreement have since stumbled. The warring parties do not trust each other. Over the past year Mr Griffiths has also had to refute claims of bias from both sides. As can become common in diplomacy, events on the ground – in Aden last month, and the attacks on Aramco oil facilities in Saudi Arabia – have complicated his task. On Monday, Mr Griffiths conceded that Yemen was at risk of being dragged into a regional conflagration between Saudi Arabia and Iran, which backs the Houthis. US President Donald Trump has pledged to back Riyadh, but he does not want a war.<br/> As a result, when the UN General Assembly meets in New York, countries will call for de-escalation in Yemen. Although violence has decreased in Hodeidah because of the ceasefire, the truce does not apply to the rest of the country. Mr Griffiths retains wide support but the lack of a grand bargain for the parties vying for control of Yemen remains the Achilles heel in the UN-led talks. An Arab coalition led by Saudi Arabia first intervened in the war in 2015, in support of Yemen's internationally recognised government. But the conflict is in a stalemate. The special envoy's mandate is also beginning to look inadequate. Not only is he caught in the middle of a security crisis beyond Yemen's borders but the unity of the country itself is in question. Violence in Aden last month, where the secessionist Southern Transitional Council seized state institutions, is now the subject of a separate mediation effort led by Saudi Arabia. The STC has thus far been excluded from the UN-led peace process in Yemen, but it is possible that the General Assembly will see calls for the separatists to be given a place at the table in any talks about Yemen's future. Several members of the UN Security Council believe this is necessary.<br/> Until that happens, however, Mr Griffiths will grapple with an unquestionable deterioration in relations between the Houthis and all other parties involved in ending Yemen's war. Despite the US saying there is evidence to the contrary, the rebels claimed they launched last week's attacks on the Aramco oil facilities. As Mr Griffiths said: “The fact that Ansar Allah [the Houthis] has claimed responsibility is bad enough. And whatever we will discover about the attack, it is a sure sign that we are moving even further away from the peace we all seek.” Yet he remains tasked with finding that elusive peace. Much like his special envoy counterpart in Syria, the Norwegian diplomat Geir Pedersen, Mr Griffiths' role can only be as effective as the flexibility he has to move the peace process forward. The likely focus of the General Assembly will be President Trump's remarks on Iran, after Washington's accusations that Tehran was complicit in the Aramco attacks. What follows, be it a military response or de-escalation, will determine if Mr Griffiths' task becomes even harder.