2000 \*\*\*\*\*. Received 2010 \*\*\*\*'
title: ' A multifrequency study of the active star forming complex NGC6357. I. Interstellar structures linked to the open cluster Pis24'
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ISM:  regions – ISM: individual objects: NGC6357 – clusters: open clusters – open clusters: individual: Pis24

Introduction
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NGC 6357 ($\equiv$ W22 $\equiv$ RCW131 $\equiv$ Sh2-11) is a large  complex located in the Sagittarius spiral arm. The  complex consists of an incomplete large shell of about 60 in diameter, many other bright optical nebulosities in different evolutionary stages, many OB stars belonging to the open cluster Pis24, and bright infrared (IR) sources, some of them young stellar object (YSO) candidates [@lortet84; @felli90; @bohigas04; @wang07; @russ10].

@lortet84 showed that the large shell was a low-excitation and ionization bounded  region. This shell, which can also be identified in images in the mid-IR [@wang07], opens to the north. The shell shows no evidence of expansion motions in optical lines [@lortet84] and has been interpreted as an ionized gas bubble created by the strong winds of the current massive stars in Pis24 or by a previous generation of stars [@lortet84; @bohigas04; @wang07].

The two brightest  regions in the complex are G353.2+0.9 and G353.1+0.6. G353.2+0.9 is the 