SDEROT, Israel, June 2 (Reuters) - Israel has deployed an Iron Dome rocket interceptor outside a Gaza border town that has borne the brunt of Palestinian shelling attacks, posing a new test for the fledgling system underwritten by Washington.

Rolled out in March in an accelerated production schedule, Iron Dome won plaudits from U.S. National Security Adviser Tom Donilon for downing eight out of nine Katyusha-style rockets fired at two southern Israeli cities from Gaza in one day.

The movement this week of an Iron Dome unit to Sderot, within four kilometers (two miles) of the Gaza Strip, signaled readiness to deal with short-range rocket and mortar barrages in the face of skepticism from some independent experts about its capabilities.

Iron Dome has passed field trials for threats with ranges of between four kilometers to 40 kilometers, so this deployment tests the lower-most end of that spectrum, said Uzi Rubin, a missile designer who consults for Israel's Defense Ministry.

Noting the recent ebbing in violence along the frontier of the Hamas-ruled Gaza, Rubin said: Let's hope action by Iron Dome is not required. But as a rule, Israel does not bring systems that are not operational into a war zone.

Disclosing the deployment on Thursday, a military source said it was part of a rotation of Israel's two Iron Domes while more of the $50 million batteries are prepared.

Israel wants between 10 and 15 units to defend its Palestinian and Lebanese fronts.

The United States said last week it planned to help Israel buy four new Iron Domes after budgeting $203.8 million in congressional funding assistance for the system in fiscal 2011.

Seeing Iron Dome sent first in March and April to Beersheba and Ashkelon, residents of Sderot had accused the government of neglecting their defenses in favor of the two industrial cities whose inland locations were harder to hit from Gaza.

(Writing by Dan Williams; Editing by Jon Boyle)