The province prevailed at a rain-soaked Parc y Scarlets with Paddy Jackson landing four penalties to add to his conversion of Andrew Trimble's ultimately decisive try.

The result saw Ulster extend their winning streak to 12 matches this season - ten of them coming in the league where they now have a nine-point lead at the top of the table.

They also had the added bonus of captain Johann Muller and Rory Best making their returns from injury, ahead of the back-to-back Heineken Cup encounters with Northampton Saints

The under-strength Scarlets, missing their Wales internationals, were unable to threaten the Ulster try-line and relied on out-half Aled Thomas for all of their points.

Both Thomas and Jackson missed early penalties as the cold and wet conditions made for a stop-start opening, with control of possession proving problematic.

Ulster edged ahead in the ninth minute, Jackson's solid connection seeing him float a 40-metre penalty through the posts.

There were very positive signs for Ulster in the scrum, their front row marching forward in a couple of early set pieces with John Afoa particularly prominent.

Kieran Murphy and Trimble stood out for some stellar high-fielding, but Craig Gilroy erred when he was caught offside as he chased a Paul Marshall kick.

Thomas took the opportunity to curl the right-sided penalty over, bringing the Scarlets level with Ulster yet to really get into gear.

Mark Anscombe's men have been slow starters in their matches this season and a spate of penalties against them invited the Welsh region forward.

Ulster were comfortable in defence though and they struck for the opening try in the 28th minute, with Trimble creating and finishing it.

Eager to impress after missing Ireland's win over Argentina, the big winger pounced on a loose Gareth Owen pass - juggling it at first - to accelerate away over halfway and in under the posts.

Jackson converted from in front of the posts and he booted the visitors 13-3 after another strong scrum from the visitors.

The error count increased for both sides as half-time loomed, the Scarlets guilt of some sloppy lineouts and rucks and Ulster spilling balls in advanced positions.

Darren Cave knocked on as Ulster threatened on the right and a poor Gilroy kick gave possession straight back to the Scarlets.

Still, the table toppers were playing the better rugby and Luke Marshall, Iain Henderson, Roger Wilson and Robbie Diack all carrying well as they looked for a late breakthrough.

Tommy Bowe and Dan Tuohy were introduced on the resumption, the latter coming in for Muller who got a welcome 40 minutes under his belt after time out with a thumb injury.

Jackson tagged on three more points from the 10-metre line, with the Scarlets pinged for hands in the ruck.

Thomas then converted the second of two penalty attempts as the Scarlets mounted a good response, upping their physicality at the breakdown and getting the crowd involved.

Andy Fenby, the scorer of their match-winning try at Munster last Sunday, came more into it as the passes began to stick for the Welshmen.

An Ulster scrum infringement led to Thomas' third successful penalty and suddenly the gap was down to a converted try.

Lively number 8 Wilson carried with relish, getting over the gain-line on a number of occasions to help Ulster get back into scoring range.

Moments later, Best and Tom Court won a scrum penalty in their first involvement and Jackson clipped over the kick to the right of the posts.

Young full-back Peter Nelson popped up in attack soon after, linking with Wilson whose pass to the supporting Mike McComish was ruled forward just as the Scarlets looked vulnerable.

The loss of Tuohy, who was sin-binned after repeated ruck offences from Ulster, hampered the visitors but Thomas sent his resulting penalty kick wide.

In a hectic spell, replacement Aled Davies ruined a break from deep by being intercepted by Court and Paul Marshall's well-weighted box kicks gave Trimble and Nelson the chance to show their ability in the air.

With man-of-the-match Wilson leading by example, the Ulstermen fended off the Scarlets' late advances as they completed a clean sweep of away victories over the Welsh regions this season.

However, the Llanelli outfit gained a losing bonus point when Thomas was successful with an injury-time penalty.