* U.S. ambassador says Iran helping Shi'ite militants and others
* Says Tehran taking advantage of Yemen's instability
* Yemenis vote for new president amid political, economic turmoil
By Tom Finn
SANAA Iran is becoming more active in Yemen and could pose a deeper threat to its stability and security, the U.S. envoy to Yemen said on Monday, highlighting what would be yet another layer of uncertainty in a near-failed state.
U.S. Ambassador Gerald Feierstein's warning is likely to reinforce long-held fears among Sunni Gulf monarchies that Shi'ite Muslim power Iran is trying exploit regional unrest.
"We do see Iran trying to increase its presence here, in ways that we believe are unhelpful to Yemen's stability and security," Feierstein said in an interview one day before Yemenis head to the polls to elect a new president to replace Ali Abdullah Saleh, ending his three decades in power.
The election, where Vice President Abd-Rabbu Mansour Hadi is the sole candidate, is part of a power transfer plan backed by the United States and brokered by Gulf Arab countries after a year of protests against Saleh's rule.
"I think that we are seeing increasing Iranian outreach to various actors," Feierstein said.
Washington is leading international efforts to isolate Iran over the Islamic Republic's nuclear program which many countries believe is aimed at building nuclear weapons, a charge Tehran denies.
Top oil exporter and close U.S. ally Saudi Arabia has accused Iran of fomenting unrest among Shi'ite populations in its east and in neighboring Bahrain.
Tehran accuses Washington of seeking to exacerbate religious and political differences between the majority Shi'ite Islamic Republic and the mostly Sunni Gulf Arab states.
In addition to its political uncertainties, Yemen faces an active wing of al Qaeda, an economic crisis that has brought it to the brink of famine, a Shi'ite rebellion in the north and a southern secessionist movement.
The northern "Houthi" rebels, who draw their name from a tribal leader, control Saada province bordering Saudi Arabia, which intervened military in Yemen in 2009.
There is ongoing fighting between the Houthis, who are members of the Zaydi branch of Shi'ite Islam, and Salafis - Sunni Muslims whose puritanical creed mirrors doctrines widespread in Saudi Arabia, and classes Shi'ites as heretics.
"We do definitely see a rise in Iranian finance, efforts on the part of Iran to increase its influence not only with Zaydi Shia elements but with Sunni elements as well," Feierstein said.
"We do think that we have evidence of Iranian activities that will build up military capabilities as well. It's a relatively recent phenomenon, Iran is taking advantage of this period of political instability and loss of government control over large parts of the country."
The U.S. Ambassador also said that while there were signs that the Houthis were willing to engage in dialogue to bring some stability to the country, the group was also expanding its territory.
"But we're also concerned about conflicts between Houthis and others in the north and a fairly aggressive effort on their part to expand their territory and their control and so we hope that through this process of national dialogue they will engage politically and work in a positive way that will end this conflict."
(Writing by Reed Stevenson; Editing by Robin Pomeroy)