U.S. navy arrives in Georgia, Russian troops stay

* U.S. warship delivers aid to Georgian port

* Blast hits Georgia's main east-west railway

* Russian soldiers man checkpoint at key Georgian port

* Ukraine vows to build up defences, eyes NATO membership



By Niko Mchedlishvili

BATUMI, Georgia, Aug 24 (Reuters) - A U.S. navy warship delivered humanitarian aid on Sunday for victims of Georgia's brief war with Russia as Moscow ignored Western demands to pull its remaining troops from the tiny Caucasus country's heartland.

Russia says the residual troops are peacekeepers needed to avert further bloodshed and to protect Georgia's separatist, pro-Moscow provinces of South Ossetia and Abkhazia. Moscow withdrew the bulk of its forces from core Georgia on Friday.

On Georgia's main east-west rail line, a fuel train exploded on Sunday near the central town of Gori after apparently hitting a landmine, sending a huge plume of black smoke into the sky.

The Russia-Georgia conflict erupted on Aug. 7-8 when Tbilisi tried to retake South Ossetia. A Russian counter-offensive pushed into Georgia proper, crossing its main east-west highway and nearing a Western-backed oil pipeline from Azerbaijan.

Russian troops also moved into Western Georgia from Abkhazia, another breakaway region on the Black Sea. Hundreds of people were killed, tens of thousands displaced and housing and infrastructure wrecked in the fighting.

In Batumi, 80 kilometres (50 miles) south of another port, Poti, where Russian troops are still present, a giant crane unloaded 55 tonnes of aid from the USS McFaul for refugees.

Underscoring U.S. support for Georgia, two other U.S. ships are due to follow the guided missile destroyer to the port. The United States has already delivered some aid by military cargo plane but is now shipping in beds and food.

"The United States is our great friend. They have arrived at such a difficult time. It means we are not alone," Georgian Defence Minister David Kezerashvili told reporters in Batumi.

Russia's Black Sea fleet flagship vessel, the Moskva, is no longer in the same area, having returned to its base in Ukraine on Saturday, Russian news agencies reported.



TRADE ROUTE

Georgian officials were assessing the scale of the damage from the fuel train blast, which could potentially disrupt a key trade route for oil exports from Azerbaijan to European markets.

Prime Minister Lado Gurgenidze told Reuters by telephone: "The railway is vital not just for the Georgian economy but for the economy of neighbouring countries."

Azeri officials said oil cargos were being held up at the border with Georgia following the explosion.

The United States and Europe fear the continued Russian presence in Georgia will cement the country's ethnic partition, undermine President Mikheil Saakashvili's pro-Western government and threaten vital energy pipelines criss-crossing the country.

Russia's action has also unnerved other ex-Soviet republics.

In a clear swipe at Moscow, Ukraine's President Viktor Yushchenko said on Sunday his country -- home to a large ethnic Russian population -- must boost its defences and speed up its efforts to join NATO. Georgia also seeks to join the alliance.

Russia sees the ex-Soviet republics as part of its legitimate sphere of influence and opposes their NATO bids.

In Georgia, the West is particularly worried about a Russian checkpoint set up at the port of Poti, which lies outside the security zone Russia says is covered by its peacekeeping mandate and is hundreds of kilometres from South Ossetia.

"Putting up permanent facilities and checkpoints are inconsistent with the (ceasefire) agreement," White House spokesman Gordon Johndroe said.

Russia has deployed its "peacekeepers" at a series of posts in the Poti area and also in a buffer zone outside South Ossetia and Abkhazia, saying they are allowed under the terms of a French-brokered ceasefire deal.

But France urged Moscow on Saturday to order its forces out of Poti as soon as possible.

Though not Georgia's busiest port for oil, Poti can load up to 100,000 barrels per day of oil products, which arrive by rail from Azerbaijan. It is also the gateway for merchandise moving to Georgia, other Caucasus republics and Central Asia.



BUFFER ZONE

French President Nicolas Sarkozy's office said he and Kremlin leader Dmitry Medvedev had agreed on Saturday on the need to create an international mechanism under the auspices of the Organisation for Security and Co-operation in Europe (OSCE) to replace Russian patrols in a buffer zone south of South Ossetia.

In a conflicting account, the Kremlin said replacing Russian peacekeepers was not discussed. Russia has earlier said South Ossetians and Abkhazians would only accept Russian peacekeepers.

Pope Benedict urged Russia and Georgia on Sunday to keep their promises to resolve the crisis peacefully.

Despite repeated demands for a complete Russian pullback to positions before the conflict, the West lacks leverage over a resurgent Russia whose oil and gas it sorely needs.

U.S. officials have said the conflict could affect Russia's membership in the Group of Eight industrialised nations and its bid to join the World Trade Organisation.

The U.S. envoy to the Caucasus said Russia had inadvertently helped Georgia's bid for NATO membership with its actions.